The transformation of native beef Achilles tendon (BAT) and rat tail tendon (RTT) collagen in admixture with ethylene glycol to the amorphous, shrunken state has been investigated over the range of volume fraction v₂ of collagen from 0 to 0.83. Precision dilatometric methods employed for v₂ Ãœ0 0.08 reveal small but unmistakable latent volume changes (ca. 5 x 10^-3 cc. per g. of collagen). The transformation is partially reversible, the latent volume change being slowly recovered in part upon cooling. Return to the previous liquidus occurs within ± 2° of the original transformation temperature T_m. These observations show conclusively that the transformation is properly represented as a phase transition, involving melting of crystalline regions, rather than as a rate process. Melting temperatures T_m also have been ascertained by polarizing microscopy and, at high dilution, from the decrease in viscosity. The dependence of T_m on v₂ conforms rather well with polymer melting theory, which yields 24 ± 5 cal/g* h-1*s for the heat of fusion of collagen. A glass-type transition observed both in native collagen-diluent mixtures below T_m and in shrunken dry collagen at 95° probably is associated with freezing-in of side chain disorder. Melting of gelatin gels resembles the re-melting of collagen allowed to re-crystallize by cooling after the initial transformation. The identity of the processes involved is indicated by the observation that T_m for gelatin-glycol is within 2° of that for collagen-glycol of the same composition.