The temperature coefficient of resonant frequency Tkf is obtained from the degree of ferroelectric domains alignment in piezoelectric ceramics and the adiabatic. piezoelec-tric equations. It is expressed as a function of the temperature expansion coefficients of the crystal lattice, their remanent polarization and other parameters. When the experi-mental data of "pure" Pb(ZrxTi1-x)O3 are substituted in the theoretical expressions, curves showing Tkf as a function of the Zr/Ti composition near the morphotropic phase boundary are obtained, and the theoretical results are found to be in close agreement with experiment. Thus the effects of different poling and heat treatment processes upon Tkf have been explained successfully. A discussion is given concerning the ways and means by which the resonant frequency temperature stability of piezoelectric ceramic vibrators can be improved.