A method for measuring the V/III ratio for epitaxial growth using threshold photoemission is demonstrated. The variation in photoyield observed during reconstruction changes of InAs (001), attributed to the variation of As dimer concentration on the surface, can be used to determine the As incorporation rate, and hence the VIII ratio in situ, in a manner analogous to As-limited reflection high energy electron diffraction oscillation measurements. Although As-limited photoyield oscillations are observed and reported here, the photoyield method for determining the VIII ratio is not dependent on the measurement of oscillations. The photoyield measurement approach is still applicable at high temperatures where step flow growth competes with island nucleation and oscillations are difficult to resolve. (C) 2003 American Vacuum Society.