Poly(ethylene terephthalate) copolymers (abbreviated PEBEPT) containing 2-butyl-2-ethyl-1,3-propanediol (BEPD) for the whole range of compositions were prepared by bulk polycondensation of either terephthalic acid or dimethyl terephthalate as the diacid monomer with mixtures of ethylene glycol and BEPD. Polymer intrinsic viscosities varied from 0.4 to 0.8 dL/g with weight-average molecular weights oscillating between 25,000 and 90,000. All the copolymers were found to have an almost ideal random microstructure. Melting and glass-transition temperatures of the PEBEPTs decreased with the content in BEPD, whereas their thermal stability was barely affected by composition. The crystallinity and isothermal crystallization rate of the PEBEPTs rapidly decreased with the incorporation of BEPD. X-ray diffraction revealed that the triclinic crystal structure of PET was retained in the copolymers.