This article compares two central pillars of China’s space programme and observes that Chinese space diplomacy is not uniform regarding international scientific co-operation either in its approaches or in results. In the case of the Chinese Space Station programme, the China National Space Administration went through existing United Nations (UN) channels and successfully attracted international partners. However, the International Lunar Research Station has avoided UN channels and used national and bilateral platforms. This bifurcation in approaches and results offers an intriguing puzzle concerning international co-operation: practices of institutionalised multilateral co-operation and areas of state-centric bilateral co-operation co-exist in this case and further complicate the issue of space diplomacy. To propose a potential explanation, it is argued here that a crucial intermediate variable — institutional density — requires further theorising, as it seems to influence strategic choices about space diplomacy, which may lead to success or failure.