The mass-resolved excitation spectra of jet-cooled guaiacol (2-methoxyphenol) and 4-methylguaiacol (2-methoxy-4-methylphenol) have been measured. For guaiacol, two rotational isomers were identified. The potential barriers for internal rotation of the methoxy group were determined by molecular mechanics calculations for the ground state S-0 and by fitting to the experimental data for the first excited state S-1. The potential energy barrier for hindered rotation of the methoxy group is found to be dependent on the hydroxy group orientation with respect to the methoxy group and increases upon excitation. The potential energy barrier is described by a term V-1 due to the interaction with the hydroxy group and a term V-2 due to the interaction with the ring. When the hydroxy group is pointing away from the methoxy group, the values are V-1 = 146 cm(-1), V-2 = 580 cm(-1) for S-0 and V-1 = 238 cm(-1), V-2 = 560 cm(-1) for S-1. When the hydroxy group is pointing toward the methoxy group, these values are V-1 = 1074 cm(-1), V-2 = 550 cm(-1) for S-0 and V-1 = 1100 cm(-1), V-2 = 500 cm(-1) for S-1.