The nucleation of poly(ethylene terephthalate) (PET) by codiols and olefinic segments was studied. The codiols 1,5-pentanediol, 1,8-octanediol, 2,5-hexanediol, and 1,3-dihydroxymethyl benzene were copolymerized into PET in a concentration range of 0-10 mol %. The melting (T-m), crystallization (T-c), and glass-transition (T-g) temperatures were studied. These codiols were found to be able to nucleate PET at low concentrations, probably by lowering the surface free energy of the chain fold. However, the codiols also disturbed the structural order of the polymer, resulting in a decrease in both the T-m and T-c values. The optimum codiol concentration was found to be at around 1 mol %, which is lower than previously reported. A diamide segment N,N'-bis(p-carbomethoxybenzoyl)ethanediamine (T2T) was found to be a more effective nucleator than the codiols; however, no synergy was observed between the nucleating effect of the diamide segment T2T and that of the codiol. An olefinic diol (C-36-diol) with a molecular weight of 540 g/mol was also copolymerized into PET in a concentration range of 0-21 wt %. Only one T-g was observed in the resulting copolymers, suggesting that the amorphous phases of PET and the C-36-diol are miscible. The main effect of incorporating the C-36-diol into PET was the lowering of the T-g; thus, the C-36-diol is an internal plastifier for PET. The C-36-diol had little effect on the T-m value; however, the T-c value actually increased in the 11.5 wt % copolymer. As the T-g decreased and the T-c increased, the crystallization window also increased and thereby the likelihood of crystallization. Therefore, the thermally stable C-36-diol appears to be an interesting compound that may be useful in improving the crystallization of PET.