NiTi shape memory alloys were investigated by Rietveld refinement of neutron diffraction data at various temperatures. For an aged sample of nominal composition Ni 5 0 . 7 Ti 4 9 . 3 at 294K, B2 austenite, R-phase, and Ni 4 Ti 3 precipitates coexist, and the Rietveld refinement provides accurate lattice parameters for the phases in this microstructure. The observed rhombohedral shape deformation occurring with the formation of Ni 4 Ti 3 and R-phase in the B2 matrix is similar in magnitude and of opposite sign. Thus the strain field around a Ni 4 Ti 3 precipitate can be compensated by a single variant of R-phase.