Thermal behavior of three phases formed in the system valine—isoleucine was studied by means of temperature-resolved powder diffractometry. Samples were untreated reactants of Val and Ile and precipitate of the discrete compound V2I crystallized from aqueous solu-tion. Investigations were performed from room temperature until substance decomposition temperatures. Thermal deformations of the crystal structures were investigated for all studied phases: parameters of tensor of thermal deformations were calculated and figures of coefficients of thermal expansion of crystal structure were created.