Alkylation of phenolic compounds with olefins is an important industrial reaction to increase the chemical stability of bio-oil. In this study, the liquid-phase alkylation of phenol with linear (1-octene) and branched (diisobutylene) olefins was studied, in a batch reactor using homogeneous (sulfuric acid) and heterogeneous (Amberlyst-15) acid catalysts. Conversion and selectivity trends were compared and the effects of different operational conditions (temperature, phenol/olefin molar ratio and catalyst content) were evaluated employing factorial experimental design. The Arnberlyst-15 solids acid resin was characterized by nitrogen physisorption and Bronsted acid sites titration. The heterogeneous catalyst showed satisfactory conversions with both olefins, albeit being less active than sulfuric acid. Higher conversions were obtained with diisobutylene than with 1-octene. C-alkylation was dominant in the diisobutylene reaction, including some product formation resulting from fragmentation of the diisobutylene olefin. However, mainly O-alkylation was observed under mild reaction conditions in the 1-octene reaction. Factorial experimental design showed that temperature was the variable that most influenced the conversion and selectivity trends, and the highest yields of di- and tri-alkylated products were obtained at elevated temperatures. A mechanistic model was proposed for both reactions. (C) 2016 Elsevier B.V. All rights reserved.