Using reflectance–difference spectroscopy to follow surface coverage on a submonolayer scale, alternating trimethylgallium (TMG) and arsine flows to separate their surface‐chemical effects and to eliminate gas‐phase interactions, and low (330–430 °C) temperatures to eliminate gas‐phase pyrolysis, we isolate and identify the different factors that kinetically limit organometallic chemical vapor deposition (OMCVD) growth on (001) GaAs by their time, pressure, and temperature dependences. When an AsH3‐stabilized surface is exposed to TMG, the relative coverage by Ga‐containing reacted species increases linearly in time to 80% coverage and exponentially thereafter, and exhibits a pressure dependence of classic Langmuir adsorption‐isotherm form. We derive a model that quantitatively describes these features in terms of reversible excluded‐volume chemisorption and subsequent irreversible decomposition of TMG at (001) surface lattice sites. Analysis of the data yields −26 and 39 kcal/mol for the chemisorption en...