The effect of phenol, o-cresol, rn-cresol, and p-cresol on the polymerization of styrene in solution and emulsion was studied.It was stated that the inhibition effect is caused by remaining oxygen in these system. I n the case of polymerization under oxygen-free conditions ( i e . vacuum), no inhibition and retardation effect was seen. The chain transfer constants, Ctr, became smaller in the order: o-cresol > p-cresol > rn-cresol > phenol. It was assumed that there exists a linear relationship between C,, and HAMMETT'S o because the phenolic substances are little dissociated. I n other words, it was considered to follow the order of the value of pKa. I n the case of co-existence of oxygen and phenolic substances, no retardation effect was seen, but inhibition effect was noticed, while, the relation, [I.P.] = k phenol^]".^, was recognized between the concentration of each phenols and induction period [I.P.]. Also, the relationship between the value of k and HAMMETT'S o was plotted along with the regression line defined by following equation: log k/k, = po and p value was -2.5. The results were discussed. ZUSAMMENFASSUNG: Es wurde die Wirkung von Phenol, 0-, rn-und p-Kresol auf die Polymerisation von Styrol in Losung und Emulsion untersucht. Noch anwesender Sauerstoff inhibiert. Bei Ausschlufl von Sauerstoff (d. h. i.Vak.) zeigte sich keine Inhibierung oder Verzogerung. Die Ubertragungskonstante Ctr wird entsprechend folgender Reihe kleiner : o-Kresol > p-Kresol > rn-Kresol > Phenol. Es wurde eine lineare Abhangigkeit der Ct,-Werte vom HAM-METTschen o angenommen, da die Phenole nur wenig dissoziieren; d. h., es wurde eine Abhangigkeit von pKa in Betracht gezogen. Phenole in Anwesenheit verzogerten die Polymerisatiori nicht, inhibierten aber gemaI3 der Gleichung [I.P.] = k . [Phenol]".'; I.P. = Induktionsperiode. Die Gleichung log k/k, = p.0 ist mit dem Wert fur p = -2,5 erfullt. Die Ergebnisse wurden diskutiert.