Two different short period superlattice (SPS) structures with nominally equivalent lattice mismatch, InAs/AlAs and InAs/GaAs are examined using in situ scanning tunneling microscopy (STM). Depending upon the growth conditions, the composition of the InAs/AlAs SPS structure can be either homogeneous or modulated in the lateral direction. Distinct periodic structures are clearly visible in images of the modulated SPS while no periodicity is observed in the homogeneous SPS. For a 30 period SPS consisting of 2 monolayers of AlAs and 2 monolayers of InAs we observe structures 20 nm in size with an average spacing of ∼25 nm in orthogonal directions, which is approximately the same length scale as the composition modulation. Despite the nominally equivalent lattice mismatch, the InAs/GaAs SPS structures are quite different. Some degree of modulation is always observed in these structures. Homogeneous structures are not observed. For the modulated SPS structures, scanning tunneling spectroscopy can be used to characterize the chemical profile.