By differential scanning calorimetry (DSC), the thermal behavior of polytetrafluoroethylene (PTFE) and random fluorinated copolymers of tetrafluoroethylene-containing hexafluoropropylene (FEP copolymers) or perfluoroalkylvinylether (PFA copolymers) as comonomers was investigated. Rapid-melt crystallization was employed to provide new data about the problem of inclusion/exclusion of co-units from the homopolymer crystal lattice. Equilibrium melting points were determined and tested in light of random copolymer predictions. Both nonequilibrium and equilibrium behaviors seem to point to the inclusion of -CF3 side groups and the exclusion of larger ones. Finally, a new value of the equilibrium melting point of PTFE is given, in good agreement with those present in the literature.