Ultra high-vacuum studies of optical second-harmonic generation (SHG) from well-characterized Si(111)-Ga interfaces in the submonolayer coverage regime are reported. Both intensity and phase measurements of the SH signal have been made. The chi _xxx and chi _xxx susceptibility tensor components are shown to have resonant enhancement around a third of a monolayer, for 1064 nm excitation. The origin of this behaviour is discussed in terms of the structure and bonding at the interface.