The kinetics of the conversion of monazite type LaCrO4 to perovskite type LaCrO3 have been investigated by isothermal TG. The kinetics are dependent on the partial oxygen pressure and the conversion reaction is clearly suppressed with increase of pressure. The appropriate model functions for the kinetics are determined based on the Avrami-Erofe’ev equation. In nitrogen, the rate determining step is a three-dimensional phase-boundary reaction (R(3)(alpha) function) and the activation energy E(a) is estimated to be about 198 kJ mol(-1). In oxygen, three-dimensional diffusion of oxygen through the LaCrO3 layer is the rate determining step (D-3(alpha) Jander function) with E(a) of about 506 kJ mol(-1). For the latter case deoxygenation might proceed via an intermediate state in which oxygen adsorbs on the LaCrO3 surface.