The concentrations of forming defects during phase transitions (PTs) and elastic coefficients in the Cu[Formula: see text]Ni[Formula: see text]S compound were determined based on differential thermal analysis (DTA) data [Formula: see text]. The sample was studied under the influence of [Formula: see text] irradiation (at a dose of 25 Mrad) and it was shown that the [Formula: see text] curves coincide each other before and after irradiation. It was established that [Formula: see text], [Formula: see text], [Formula: see text] modifications are commensurate when heating the sample. A certain number of defects in S atoms at lattice sites and defects in Cu atoms (also Ni atoms) in interstitial sites of the sublattice are some of the causes of lattice rearrangement.