We have developed an empirical model of AlAsySb1-y composition during molecular beam epitaxy as a function of growth temperature, growth rate, As-2-flux, and Sb-2-flux, using response surface methodology. In order to facilitate data collection from a large number of experiments, we have applied desorption mass spectroscopy as on in situ measure of alloy composition. A quadratic model is found to fit the data well, identifying trends and several interactions between growth parameters. Published by Elsevier Science B.V.