Abstract Local distortions of the sphalerite crystal structure of Zn 1‐ x Co x S, Zn 1‐ x Co x Se ( x =0.01) semiconductor crystals were studied for the first time using thermal neutron diffraction at 300 and 78 K. The diffraction patterns of the crystals were revealed to contain diffuse scattering regions concentrated in the vicinity of the strong Bragg reflections. It was detected for both crystals that the intensity of the neutron diffuse scattering essentially decreases with cooling. Some arguments are given to explain the revealed properties of the diffuse scattering for these crystals in terms of a formation of the vibronic effect leading to the emergence of local nanosized distorted regions. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)