Thomson then moved on to Phnom Penh and took photographs of the King of Cambodia and other members of the Cambodian Royal Family, before travelling on to Saigon. From there he stayed in Bangkok briefly, before returning to Britain in May or June in 1866. While back home, Thomson lectured extensively to the British Association and published his photographs of Siam and Cambodia. He became a member of the Royal Ethnological Society of London and was elected a Fellow of the Royal Geographical Society in 1866, and published his first book, "The Antiquities of Cambodia", in early 1867.