We report studies of the kinetics of thermal decomposition of triethylgallium (TEGa), trimethylgallium (TMGa), and trimethylindium (TMIn) adsorbed on GaAs(100) in ultrahigh vacuum. The adsorbed layers were prepared by dosing GaAs(100) at room temperature, to either saturated coverage or coverages below saturation. The relative coverage of carbon was monitored by x‐ray photoelectron spectroscopy (XPS) as the substrate temperature was slowly increased (0.6–3.2 °C/min). Products were detected at faster heating rates (0.7–6 °C/s) with a differentially pumped quadrupole mass spectrometer. The substrate temperature was measured by infrared laser interferometric thermometry. The kinetic analysis also makes use of XPS and mass spectrometric data on laser‐induced, rapid thermal decomposition (heating rates of ∼1011 °C/s ). TEGa dissociatively chemisorbs on GaAs(100) at room temperature. Heating the substrate from room temperature to ∼500 °C results in desorption of a Ga–alkyl at low temperature, ascribed mostly to diethylgallium (DEGa) and possibly some TEGa.At higher temperature, C2H4and C2H5desorb in parallel after most of the Ga–alkyl has desorbed. The hydrocarbon desorption is described well by simple first order kinetics with an activation energy,Eact=32±4 kcal/mol, and a pre‐exponentialAfactor of 2.5×1010±1.5s−1. Ga–alkyl desorption is more complicated; the Arrhenius parameters for assumed first order desorption exhibit strong coverage dependences. A fit to all the data was obtained forA=5×108s−1andEact=18 kcal/mol at saturated coverage, with a large decrease inEact(or increase inA) with decreasing coverage. TMGa decomposes to yield a Ga–alkyl desorption product (either dimethylgallium, TMGa, or a mixture of the two) at low temperature, and CH3at higher temperature. CH3desorption has a first order activation energy of 43±2 kcal/mol for an assumedAfactor of 1×1013s−1. For the Ga–alkyl,A=108s−1andEact=19 kcal/mol, with a coverage dependence similar to DEGa desorption from TEGa decomposition. TMIn undergoes a methyl exchange reaction on GaAs(100). Upon heating above room temperature, a Ga–alkyl desorbs first, followed by desorption of CH3at higher temperature. The Ga–alkyl has with the same cracking pattern as observed for TMGa decomposition. No In–alkyls desorb, and In desorption does not occur until all carbon‐containing species desorb. CH3starts to desorb at lower temperature than for TMGa decomposition. Assuming anAfactor of 1×1013s−1, CH3desorption over the observed wide temperature range indicates a range of activation energies from 33–43 kcal/mol. Ga–alkyl desorption is similar to that observed during TMGa decomposition. At saturated coverage,A=108andEact=17 kcal/mol. However, the coverage dependence is not as strong as for TMGa, so that Ga–alkyl desorption peaks at lower temperature for TMIn. Decomposition mechanisms for these group‐III metal alkyls are discussed, along with implications for growth of III–V compound semiconductor films from these precursors by chemical vapor deposition and molecular beam techniques.