We present a study of the structural and optical properties of a heterostructure emitting in the midinfrared. The structure consists of monolayerlike InSb quantum wells inserted in an InAs/GaSb superlattice (SL) matrix. X-ray diffraction and transmission electron microscopy analyses show a high structural quality of the structure. A strong emission line with a peak energy near 0.30 eV (3.5 μm) is observed from the monolayerlike InSb. In order to identify the physical origin of this transition, excitation density and temperature dependent photoluminescence experiments have been performed on samples with different nominal InSb thicknesses and SL designs. The experimental results suggest a type II band alignment, with electrons localized in the conduction miniband of the InAs/GaSb SL matrix and holes localized in the monolayerlike InSb. This assignment is supported by the shift of InSb layer emission to lower energies when the SL design is changed, and by tight-binding calculations.