Atomic force microscopy was used to study changes in the surface step structure of GaInAsSb layers with varying degrees of phase separation. The layers were grown by organometallic vapor phase epitaxy on (0 0 1) GaSb substrates with 2 degrees miscut angles toward (1 1 1)A, (1 1 1)B, and (1 0 1). Alloy decomposition was observed by contrast modulations in plan-view transmission electron microscopy, and broadening in X-ray diffraction and photoluminescence peaks. GaInAsSb layers with a minimal degree of phase separation exhibit a step-bunched step structure. A gradual degradation in the periodicity of the step structure is observed as the alloy decomposes into GaAs- and InSb-rich regions. The surface eventually develops trenches to accommodate the local strain associated with composition variations, which are in the order of a few percent. The surface decomposition is affected by substrate miscut angle, and although phase separation cannot be eliminated, its extent can be reduced by growing on substrates miscut toward (1 1 1)B.