The Joint Committee of Exchanges and Cooperation between Seoul and Beijing (hereunder the “Seoul-Beijing Joint Committee”) was founded in 2013 to mark the 20th anniversary of the establishment of a sister city relationship between the two cities. The Committee aimed at establishing itself as a platform for sustainable exchanges and cooperation between the two cities based on several factors. Based on experience, a shift away from one-off exchange towards greater and sustainable exchange and cooperation between the capital cities of South Korea and China was envisioned, prompted by their increasing role in non-traditional and transnational security issues as well as stronger economic ties. 
The Seoul-Beijing Joint Committee, which will hold its fourth conference in May 2021, has been carrying out various projects for exchange and cooperation in four major areas including economy, culture, education and environment with a view to: i) establishing a sustainable exchange platform, ii) planning and managing agenda for the pursuit of common interests, and iii) building an expertise-based human resource network and system. 
“Sustainability”, the focal point of the Seoul-Beijing Joint Committee, is directly related to and proportional to the degree of institutionalization of the committee. This report reviews the degree of institutionalization of the committee from three aspects: “in-group identity”, “generalized principles of conduct”, and “indivisibility and diffuse reciprocity”. 
First, “in-group identity” is assessed by gradual development within the committee. Faced with adverse external circumstances, such as THAAD deployment and the Covid-19 pandemic, the two cities postponed a scheduled conference instead of cancelling it altogether, thereby continuing to host conferences. Since its founding in 2013, the committee has met regularly every two to three years. The organization has expanded from three to four teams (Economy, Culture, Education, and Environment). Its regular conference was upgraded to mayoral level from the second meeting, resulting in increased numbers of official participants and higher-level officials in the secretariat and the respective teams.
The committee showed improvements in “generalized principles of conduct”, which are being followed by signing of MOUs and the promotion and implementation of exchange and cooperation. A total of seven MOUs have been concluded by the third conference held in 2018. Since the first conference, the promotion and implementation rates have climbed by 21.9% and 5.4%, respectively, in the second conference, indicating a higher tendency for adherence to consensus between the two sides. 
“Indivisibility and diffuse reciprocity” appears to have emerged during the promotion and implementation of “mutual” exchanges and plans for cooperation as well as their retention in the pursuit of common interests. The rate of promoting “mutual” plans rose to 78.0% in the second conference compared with 59.5% in the first conference. The rate of plan retention also increased to 62.1% in the third conference from 39.2% in the second conference, showing signs of a positive change. 
Overall, the Seoul-Beijing Joint Committee appears to have evolved into a solid model of city diplomacy between the two cities with a focus on common interests. The three elements of “in-group identity”, “generalized principles of conduct” and “indivisibility and diffuse reciprocity” are creating a virtuous cycle, reinforcing each other. Despite issues such as lack of in-group identity for the Education Team due to the absence of a mutual regulation mechanism and a temporary decline in cultural and economic exchanges and cooperation caused by THAAD deployment, the committee has achieved a certain level of “institutionalization”.
The development of the committee into a sustainable model of city diplomacy requires formulation of clear initiatives to enhance the level of institutionalization. Such initiatives can include: i) developing a network of cities in Northeast Asia to cope with common issues in the longer term, ii) enhancing health security of the citizens of the two cities by establishing a Health and Medical Team in the near term, iii) signing MOUs between different teams for conference and systematic assessment of exchanges and cooperation, iv) developing initiatives for “mutual” exchange and cooperation, and v) managing plans for near- and long-term exchange and cooperation separately.