The transformation of m- and o-cresol has been investigated on a series of HY zeolites with Si/Al ratio in the range of 4.5 to 55, at 380 degrees C and atmospheric pressure. It was found that the zeolite activity increased but stability decreased with N-Al for both m- and o-cresol. For Si/Al = 4.5 the zeolite activity was similar for both reactants, but as Si/Al ratio increased, o-cresol became relatively more reactive than m-cresol, and for all Si/Al ratios the catalysts were more stable in the m-cresol transformation. Both mand o-cresol were converted mainly through two parallel reactions - isomerization and disproportionation. The disproportionation reaction was predominant, and more important in the case of o-cresol. The isomerization/disproportionation selectivity increased for m-cresol and decreased for o-cresol with N-Al. Catalyst deactivation favoured disproportionation in the case of o-cresol and isomerization in the case of m-cresol, except for o-cresol conversion on HY(4.5), where both reactions were equally affected. Isomerization selectivity for m-cresol (p/o) was found to be higher than the equilibrium value and to increase with catalyst deactivation; while for o-cresol, the isomerization ratio (m/p) did not change with time on stream for HY(4.5) and HY(16.6) and decreased for HY(55). Disproportionation selectivity (ph/Sigma x) was equal to unity independently of Si/Al ratio or catalyst deactivation.