This paper deals with phase transitions of niobium-doped PZT ceramics. Phase diagrams of PbZr 1- xTi xO 3 + yNb 2O 5 for x = 0,035 and 0,045 with 0 < y < 0,01 are given. An apparatus for remnent polarization measurement is described and experimental conditions are specified. Experimental results are interpreted with the help of a modified molecular field theory in which the molecular field coefficient is supposed to be a function of electric polarization. A discussion on phase transition order and on the theory validity is undertaken.