Poly(ethylene terephthalate)/poly(butylene terephthalate) blends [xPET/(l-x)PBT] were prepared by ultraquenching of the melt between two metal rolls rotating in a liquid nitrogen bath. Both DSC and WAXS studies indicate that immediately after preparation, the blends are amorphous regardless of the composition. Two glass transition temperatures are found for the as-quenched 0.5 PET/0.5 PBT blend. The activation energies determined from the dependence of each glass transition temperature on the heating rate are approximately the same. Furthermore, they are equal to the mean activation energy for the overall crystallization. This can be explained in terms of the percolation threshold theory.