Optical-absorption measurements have been carried out on tensile and compressive In x Ga 1 − x As/InP strained layers. It is shown that the energetic dispersion of the heavy-hole relative to the light-hole subband σ HH / σ LH is the key to knowing the origin of the microscopic inhomogeneities. So, σ HH / σ LH <1 indicates the existence of composition inhomogeneities whereas σ HH / σ LH =2.8 reveals an inhomogeneous strain field.