Stress-to-strain differential values were measured in tensile testing at various stages of deformation in both austenitic and martensitic states of a near-equiatomic NiTi shape memory alloy as a measurement of the moduli of elasticity for the two phases. It was found that the measurements varied with testing conditions, including strain and temperature. This observation indicates that the mechanical measurements are only the apparent modulus of elasticity, which is affected by contributions to the apparent elastic deformation from other deformation mechanisms. These additional contributions to deformation are attributed to the stress-induced martensitic transformation and martensite reorientation processes. (C) 1998 Elsevier Science S.A.