The interaction of H-2 with GaAs(100) surfaces has been monitored using optical second-harmonic generation (SHG) excited using an Nd:YAG laser operating on the sub-bandgap fundamental line at 1064 nm. The SHG response has been recorded both before and after treatment of the substrate in H-2 to remove contamination and under a high overpressure of H-2 (10 mbar). It is found that the polar SHG response from a GaAs(100) surface contaminated with carbon and oxygen is essentially structureless apart from the appearance of a peak in SHG intensity when the incident electric vector is aligned with a <110> azimuth. After cleaning in H-2, the polar SHG intensity response is both more intense and more structured, showing maxima corresponding to the situation in which the incident electric vector is aligned with both the <110> and <-110> azimuths. No further changes are observed in the polar SHG response when measurements are made with the GaAs(100) surface under 10 mbar H-2. While these data are preliminary and may not be directly interpreted in terms of a particular two-dimensional surface overlayer, they illustrate the high surface SHG response obtainable from GaAs(100), which is discussed in terms of the presence of a highly non-linear surface layer.The interaction of H-2 with GaAs(100) surfaces has been monitored using optical second-harmonic generation (SHG) excited using an Nd:YAG laser operating on the sub-bandgap fundamental line at 1064 nm. The SHG response has been recorded both before and after treatment of the substrate in H-2 to remove contamination and under a high overpressure of H-2 (10 mbar). It is found that the polar SHG response from a GaAs(100) surface contaminated with carbon and oxygen is essentially structureless apart from the appearance of a peak in SHG intensity when the incident electric vector is aligned with a <110> azimuth. After cleaning in H-2, the polar SHG intensity response is both more intense and more structured, showing maxima corresponding to the situation in which the incident electric vector is aligned with both the <110> and <-110> azimuths. No further changes are observed in the polar SHG response when measurements are made with the GaAs(100) surface under 10 mbar H-2. While these data are preliminary and may not be directly interpreted in terms of a particular two-dimensional surface overlayer, they illustrate the high surface SHG response obtainable from GaAs(100), which is discussed in terms of the presence of a highly non-linear surface layer.