The influence of CO molecular adsorption on the thermodynamically favoured atomic-scale structures of Au nanoparticles is investigated theoretically. Nanoparticle free energies are calculated using an embedded atom model potential in conjunction with a simple statistical mechanics model allowing nanoparticles containing up to 10 000 atoms (7 nm diameter) to be simulated. The sequence of high stability morphologies with increasing number of atoms is modified from (decahedral --> icosahedral --> Marks-decahedral --> truncated octahedral) in vacuum to (octahedral --> Marks-decahedral --> truncated octahedral) under gas pressure. As well as being of fundamental interest, these effects are relevant to applications in gas sensing and heterogeneous catalysis.