Phase relations in (Pb, La)Zr0.55Ti0.45O3 ceramics have been investigated by means of X‐ray diffraction and dielectric measurements as functions of the La concentration and temperature. Between a tetragonal, ferroelectric phase (FE) and a cubic, paraelectric phase (PE), in the phase diagram a pseudo‐cubic phase, a so‐called quasi‐ferroelectric phase (QFE) was found. The phase boundary FE-QFE found from dielectric and X‐ray data approximately coincides, while the unit cell volume and unit cell dimensions show discontinuities at this boundary. The dielectric properties were investigated in the QFE and PE phase fields as functions of temperature and frequency. The dielectric constant ϵ as a function of temperature could be represented in both the phase fields by a quadratic law of the type ϵ−1 = ϵmax−1 + C(T-T'c)2, while T'c as well as tan δmax were dependent on the frequency.