Abstract Using a wedge type cell, we have studied the SA–S*c, S*c–Sc and SA–Sc phase transition lines in the ferroelectric liquid crystals 4-(2-methylbutyl)phenyl 4′-n-octylbiphenyl-4-carboxylate (CE-8) and 2-methylbutyl 4-(4-decyloxy-benzylidene)aminocinnamate (DOBAMBC) as a function of cell thickness in the planar geometry. The phase diagram is similar to the one observed using an external magnetic field. A surface induced re-entrant-like phenomenon is also observed. The experimental results are compared with the predictions of a Landau model with surface anchoring and a qualitative agreement was found. The results show that whereas disclination lines are not important for the SA–S*c and SA–Sc transition lines, these defects have to be taken into account when evaluating the S*c→Sc transition line. The form of the phase diagram has some important implications for the performance of surface stabilized ferroelectric liquid crystal cells used in electrooptic devices.