The removal of surface carbon and oxygen contamination from GaAs (100) surfaces via the interaction of H-2 has been monitored using optical second harmonic generation excited using an Nd:YAG laser operating on the sub-bandgap fundamental line at 1064 nm. The total SHG response as a function of rotation of the plane of polarisation of the incident radiation with respect to the surface has been recorded both before and after treatment of the substrate in H-2 to remove contamination. It is found that SHG data for a GaAs (100) surface contaminated with carbon and oxygen exhibits a high degree of anisotropy in the SHG signal, in that the total SHG intensity response is maximised when the plane of polarisation of the incident radiation is aligned along the <110> surface azimuth. After cleaning in H-2, a second maximum in the total SHG response is observed corresponding to the case where the incident electric vector is aligned along the orthogonal <110BAR> surface azimuth. The data are discussed in terms of the potential of the SHG technique for the monitoring of the removal of surface contamination prior to surface processing.The removal of surface carbon and oxygen contamination from GaAs (100) surfaces via the interaction of H-2 has been monitored using optical second harmonic generation excited using an Nd:YAG laser operating on the sub-bandgap fundamental line at 1064 nm. The total SHG response as a function of rotation of the plane of polarisation of the incident radiation with respect to the surface has been recorded both before and after treatment of the substrate in H-2 to remove contamination. It is found that SHG data for a GaAs (100) surface contaminated with carbon and oxygen exhibits a high degree of anisotropy in the SHG signal, in that the total SHG intensity response is maximised when the plane of polarisation of the incident radiation is aligned along the <110> surface azimuth. After cleaning in H-2, a second maximum in the total SHG response is observed corresponding to the case where the incident electric vector is aligned along the orthogonal <110BAR> surface azimuth. The data are discussed in terms of the potential of the SHG technique for the monitoring of the removal of surface contamination prior to surface processing.