All nine independent elastic constants have been determined for a biaxially stretched poly(ethylene terephthalate) (PET) film using novel mechanical methods. The orthotropic directions and the in-plane Poisson's ratios were first characterized using vibrational holographic interferometry of tensioned membrane samples. The out-of-plane Poisson's ratio was obtained by measuring the change in tension with the change in pressure for constant strain conditions. Pressure-volume-temperature (PVT) equipment was used to measure the bulk compressibility as well as the volumetric thermal expansion coefficient (TEC). The inplane Young's moduli were obtained by tensile tests, while the out-of-plane, modulus was calculated from the compressibility and other elastic constants that describe the inplane behavior. The in-plane TECs in the machine and transverse directions were determined using a thermal mechanical analyzer (TMA). The out-of-plane TEC was determined using these values and the volumetric TEC determined via PVT. The resulting compliance matrix satisfies all of the requirements of a positive-definite energy criterion. The procedure of characterization utilized in this article can be applied to any orthotropic film.