High-resolution core-level photoemission was used to examine the interaction between the oxidised silicon-terminated diamond (100) surface and the molecular acceptor MoO3. An observed downward shift in the Fermi level position, accompanied by the appearance of two distinct charge states of MoO3, indicates charge transfer from the surface into the MoO3 adlayer in the form of surface transfer doping with a concurrent accumulation of holes in the diamond.