In vacuo scanning tunneling microscopy is used to investigate the surface reconstructions of pseudomorphic InGaAs alloys at intermediate values of compressive strain. The coverage of different reconstructions varies with film thickness, concomitant with changes in composition and strain at the surface arising from In segregation and changes in surface morphology. Thin samples exhibit mainly disordered (1 X 3) reconstructions along with small regions of incommensurate (1 X 2). With increasing thickness, the (1 X 3) transforms into more regular (4 X 3) or c(4 X 6), whose coverage mirrors the increase and saturation of In surface composition. Regions of alpha 2(2 X 4) reconstructions are also present, and their coverage initially increases with In surface composition, but later decreases upon saturation of In at the surface. This decrease is concur-rent with the onset of surface roughening, suggesting that the alpha 2(2 X 4) reconstruction is strain stabilized. (c) 2006 American Vacuum Society.