First-principles calculations are performed to investigate alloying and ferroelectric effects in lead zirconate titanate (PZT) with high Ti composition. We find that the main effect of alloying in the paraelectric phase of PZT is the existence of two sets of B–O bonds, i.e., shorter Ti–O bondsvs.longer Zr–O bonds. On the other hand, ferroelectricity leads to the formation of very short covalent Ti–O bonds and to the formation of covalent chains of Pb–O bonds. The covalency in the ferroelectric phase is mainly induced by an enhancement of hybridization between Ti3dand O2p,and between Pb6sand O2p.These hybridizations induce a striking decrease of the effective charges when going from the paraelectric to the ferroelectric phase of PZT. ©1998 American Institute of Physics.