Using real-time reflection mass spectrometry, we have measured the temperature dependence of As2 desorption and As4 ‘‘reflection’’ from a GaAs(001) surface. The measurements agree well with the second-order flux-balance equation expected to govern surface stoichiometry, and determine its rate constant. Using that rate constant, the equation associates physically reasonable surface stoichiometries with the various well-known GaAs(001) surface reconstructions, and is strikingly consistent with measurements of the times for surface reconstruction changes in the absence of an As ambient.