Abstract: Drastic molecular weight degradation of a p-(polybutyl-substituted phenol occurs when it is heated above 65.degree. C. in the presence of BF.sub.3 -phenolate catalyst used in the preparation of said substituted phenol by the alkylation of phenol with a butylene polymer of 500 to 3000 molecular weight. Such molecular weight degradation can be substantially suppressed by providing 20 to 30 weight percent dissolved phenol in the mixture resulting from such catalytic alkylation reaction and then rapidly heating the mixture with such amount of dissolved phenol to a temperature of from 182.degree. C. up to 200.degree. C. at extant atmospheric pressure whereat the catalyst dissociates and liberated BF.sub.3 is stripped out by phenol vapors.