The cresol transformation was investigated on HZSM-5 at 380 degrees C. Isomerization and disproportionation products were observed. Isomerization was the main reaction. The cresol isomerization rates on HZSM-5 increased from ortho-to meta-to para-isomer with decreasing molecular kinetic diameter. The initial conversion rates were in a ratio r(p):r(m):r(o) = 20:3:1. The isomerization product selectivity favoured molecules with the smallest kinetic molecular diameter at the onset of the reaction, and as reaction proceeded, the reaction product composition was determined thermodynamically. For ortho-cresol, at low conversion the m/p ratio slightly decreased with conversion, and at higher conversions the selectivity became insensitive to the degree of conversion, giving a m/p ratio equal to 3. For meta-cresol isomerization, the p/o ratio decreased steadily with conversion, and the initial para selectivity was associated to both transition state and product shape/size selectivity. The isomer equilibrium composition obtained was 36% ortho-, 48% meta-, and 16% para-cresol, in disagreement with the thermodynamically calculated composition. The isomerization rates were always greater than disproportionation rates. The isomerization rates followed the sequence r(I)(p) > r(I)(m) > r(I)(o), while the disproportionation rate sequence was r(D)(p) > r(D)(m) congruent to r(D)(o). The initial I/D selectivities were in a ratio (I/D)(p):(I/D)(m):(I/D)(o) = 100:30:8.