Abstract: A process (and apparatus therefor) for carrying out thermally initiated chemical reactions or physical changes by heating a particulate substrate in a fluidized bed having hotter zone(s) and cooler zone(s) between which the fluidized particles circulate, the dwell time in the cooler zone(s) being sufficiently long to allow chemical reactions(s) or physical change(s) to take place so that when the particles move into the hotter zone(s) the undesirable transformation does not take place, and optionally the dwell time in the hotter zone(s) being sufficiently short so that other undesirable transformation(s) does not take place before the particles move into the cooler zone(s). The fluidized bed process is applicable, for example, to a process for the production of poly(ethylene terephthalate) from terephthalic acid and ethylene oxide.