Crystallization of bisphenol A polycarbonate/poly(butylene terephthalate) (PC/PBT) blends at low undercoolings (less than or equal to 60 degrees C) was first studied in isothermal conditions by rapidly cooling samples from the melt. Differential scanning calorimetry (d.s.c.) half-crystallization times (t(1/2)) were obtained. At the same time, transmission electron microscopy (TEM) analyses were undertaken on microtome sections from quenched and crystallized d.s.c. samples. The d.s.c. behaviour appears to be linked to the dispersion of PBT in the blends. For blends in which PBT is finely dispersed in PC, t(1/2) lies around 10(3) s. In the reverse situation, the blends, like pure PBT, cannot be quenched, showing a t(1/2), in the same range as that of the pure polymer. In the co-continuous range, t(1/2) measurements are not very reproducible. They lie between those of dispersed PC and dispersed PBT blends. On the basis of these observations, original mechanisms of crystallization are proposed. The main feature of these mechanisms is the slowness of the PBT crystallization when it is finely dispersed. This behaviour is attributed to a low nucleation density. Crystallization of PC/PBT blends at high undercoolings was undertaken using d.s.c. cooling experiments. When some PBT is finely dispersed, an exotherm occurs on cooling, well below the T-g of the PC-rich phase. An evolution of T-ch, Delta H-ch (respectively, crystallization temperature and enthalpy on heating) and Delta H-m (melting enthalpy) as a function of annealing temperature occurs on reheating the samples. Surprisingly enough, a stepwise transition in morphology takes place around 110 degrees C, which is confirmed by the very different microstructures observed by TEM for samples annealed at 105 degrees C and 150 degrees C. A tentative phase diagram is finally proposed which shows that a spinodal-type phase decomposition should occur around 110 degrees C. Competition between a crystallization mechanism involving a homogeneous nucleation step and this spinodal phase decomposition probably takes place. The result is that the homogeneous nucleation mechanism, which is shown to occur in pure PBT when finely dispersed, is delayed until the spinodal decomposition occurs.