The Royal Watercolour Society archives are stored at Westminster Archives Centre and are available for viewing and research on appointment. The archives contain documents tracing back to the foundation of the Society in 1804 and includes items such as meeting minutes, postcards, photographs and letters.
The Business of Watercolour: A Guide to the Archives of the Royal Watercolour Society by Simon Fenwick and Greg Smith references the contents of the archives and is the suggested place to start research.