Investigations of the structure-property relationships in Pb 1-3x/2Lax(Zr0.55Sn0.35Ti 0.10)O3·. (PLZST 100x/55/35/10) have been performed for x between 0 and 0.08 by means of x-ray diffraction, electron microscopy, polarization, and dielectric studies. Factors affecting the phase stability were determined. The base composition PLZST 0/55/35/10 was ferroelectric (FE) at room temperature and transformed into an incommensurate antiferroelectric (AFEin) state on heating. Evidence was presented that low La addition could destabilize the FE state and PLZST 2/55/35110 became AFEin at room temperature, which transformed to a multicell cubic state and then to a paraelectric (PE) phase upon heating. Further increase of La content enhanced the stability of the AFEin state at room temperature. Moreover, addition of La could also broaden the AFEin-PE phase transition temperature region and decrease its maximal dielectric constant ε′max.