The low-temperature martensitic phase transformation in FeriNija was studied by x-ray diffractometry, differential scanning calorimetry and inelastic neutron scattering. From inelastic neutron scattering spectra, the phonon density of states of the high-temperature austenite and low-temperature martensite were obtained, showing a substantial stiffening in the martensite phase. The change in vibrational entropy associated with this stiffening was found to be ΔS vib γ→α = -0.12 ±0.02 k B atom -1 . The forward and reverse transformations occur at different temperatures and the total entropies of transformation were measured by differential scanning calorimetry for both. From these measurements and published magnetization results for this alloy, it is found that the change in magnetic entropy is twice as large as the phonon contribution in the reverse martensitic transformation, accounting for the inequality of the total entropies of the forward and reverse transformations.