The catalytic performance of tungstophosphoric and molybdophosphoric acids supported on two commercial carbons, for dehydration reactions of several alcohols, was studied. The alcohol conversion depended on their molecular size, while selectivity to alkenes was also related to cyclodimer molecular size formed by certain alkenes in acidic medium. Acidity and mean pore diameter of the catalysts showed an influence on the conversion degree of the alcohols into alkenes and also into alkene cyclodimerization.