Zinc antimonides exhibit high thermoelectric figures of merit and bear great potential for power generation. To advance the understanding of the structure–property relationship in these alloys, the microstructure of a nanocrystalline Zn_4Sb_3 alloy, containing 57.29 at.% Zn, was investigated by scanning electron microscopy and atom probe tomography. Chemical inhomogeneities were observed at grain boundaries. Within the grains the distribution of Zn shows large fluctuations inducing a complex microstructure made of Zn-rich and Sb-rich regions at a nanometer scale.