We have investigated the use of tri‐isobutylgallium (TIBG) as an alternative to the standard source, triethylgallium (TEG), for growth of GaAs and AlGaAs by metalorganic molecular beam epitaxy (MOMBE). With this source we have obtained specular morphologies and growth rates up to 83 Å/min and 92 Å/min for GaAs and AlGaAs, respectively. At low temperatures, <450 °C, both GaAs and AlGaAs show much less carbon when grown with TIBG rather than TEG. At 525 °C, carbon levels in GaAs are similar regardless of Ga source. However, by optimizing the V/III ratio, the carbon background in Al0.25Ga0.75As can be reduced to 3×1015 cm−3 with TIBG relative to the 2×1016 cm−3 normally obtained with TEG at this temperature. Furthermore, the use of TIBG reduces the oxygen background by roughly a factor of 2. Based on these results, we conclude that the oxygen background in MOMBE is no longer limited by oxygen contamination in the gaseous precursors.