Thermal decomposition mechanisms have been inferred for a series of organoarsine chemical vapor deposition precursors, and this data has been correlated with the quality of GaAs films grown from these reagents. Tri‐, di‐, and mono‐ethylarsine, as well as a mixture of triethylarsenic and arsine, were pyrolyzed under pseudogrowth conditions, and their decomposition mechanisms were inferred from a qualitative and quantitative analysis of the reaction mixture components. The primary decomposition step for the ethylarsines appears to be a thermally induced, arsenic‐carbon bond homolysis to produce both an ethyl radical and an alkyl and/or hydride substituted arsenic radical species. For a mixture consisting of arsine and triethylarsenic, it appears that the triethylarsenic reagent undergoes arsenic‐carbon bond homolysis, and the radicals thus produced enhance the decomposition of the arsine coreagent. The more highly substituted ethylarsine reagents were found to generate the greatest number of alkyl‐substituted arsenic radicals upon decomposition, and also produced the least pure GaAs films. Since alkylarsenic radicals can react with a growing GaAs epilayer to cause severe carbon contamination, this decomposition data is consistent with the observed growth results. In the coreagent mixture, the free‐radical activation of arsine results in a large production of dihydridoarsenic radicals, which is consistent with the high‐purity, low‐carbon films produced from this reagent mixture. These results indicate that any viable organoarsenic precursor must decompose preferentially to produce hydrido‐arsenic radical intermediates, in order to produce high‐purity GaAs epilayers.