The glass transition temperature (T-g) of a series of statistical copolymers of vinylidene fluoride and trifluoroethylene has been determined experimentally. Its variation as a function of the chemical composition of the copolymers has been discussed based on the Fox, Di Marzio and Couchman equations. Using T-g = -42 degrees C for poly(vinylidene fluoride) and a value of T-g usually reported for polytrifluoroethylene (T-g = 31 degrees C), a significant discrepancy has been obtained between experimental and predicted values. Thus, it has been suggested that the glass transition temperature for polytrifluoroethylene is located in the range of -20 to -10 degrees C, which is consistent with Wunderlich’s thermal analysis data.