A clear ferroelectric transition of first-order type is observed in poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) random copolymers with TrFE contents higher than 20% and lower than 40%. This structural phase transition which takes place in the crystalline regions of the polymeric material has been analysed using neutron diffraction under hydrostatic pressure for three copolymer compositions : 80/20, 70/30 and 60/40. The P-T phase diagrams have been determined in the ranges 0.1-300 MPa, 300-500 K both upon heating and upon cooling, and several thermodynamic parameters have been evaluated for the different crystalline phases : compressibility and thermal expansion coefficients, enthalpies of transition, etc. For the middle composition (70/30) the ferroelectric transition temperature T-C is well separated from the melting temperature T-m of the paraelectric phase and both transition temperatures increase with increasing pressure : dT(c)/dP=0.38 K MPa(-1) and dT(m)/dP=0.25 K MPa(-1) For the 80/20 composition the Curie temperature T-C is closer to the melting temperature T-m, and under increasing pressure the two transition lines are expected to merge at a triple point (P*approximate to 500 MPa, T*approximate to 560K), above which a single line corresponds to the melting of the ferroelectric phase. For the 60/40 composition a new ’low temperature disordered’ phase appears and coexists with the ferroelectric phase but with a volume fraction which depends on temperature and thermal history.