Located on the ground floor, the Lecture Theatre and Lower Library provides an inviting setting for conferences.
The Theatre was refurbished with help from the Society's Petroleum Group in 1999, and then again for the Bicentenary in 2007. It was named the Janet Watson Lecture Theatre in 2009, in honour of Professor Janet Vida Watson FRS (1923-1985), of Imperial College London, the Society's first woman President (1982-1984).
The Lower Library was originally the Society's Main Library. It is now mainly to store foreign periodicals and is used for social gatherings and for refreshments.