We have investigated the use of tertiarybutylarsine (tBAs) as an As source in the surface-selective growth of GaAs and AlGaAs by low-pressure OMVPE. Deposition was carried out onto GaAs substrates partly masked with SiNx. Triethylgallium (TEGa) and trimethylaluminum (TMAl) were used as precursors along with either arsine or tBAs. Growth pressures were in the range 0.3 to 3.0 Torr. The usual trends of decreasing deposition on the mask with increasing temperature, decreasing group V/III ratio, and decreasing pressure were observed. At very low group V/III ratios, for either As precursor, single crystal material with a pitted surface was obtained. A reduction in the nucleation of polycrystalline material on the mask was observed when tBAs was used in place of AsH3.