The photodissociation of p-methoxytoluene and p-methoxybenzyl alcohol at 266 nm in n-heptane solution is studied by nanosecond fluorescence and absorption spectroscopy. The formation of a p-methoxybenzyl radical is identified by its fluorescence which is induced by excitation at 308 nm. The yields of the radical are of the order of similar to 10(-3) for dissociation of p-methoxytoluene and p-methoxybenzyl alcohol. The growth rate of 1.5x10(8) s(-1) for the radical is equal to the decay rate of ( 1.5 +/- 0.3) x 10(8) s(-1) for the precursor fluorescence in dissociation of p-methoxytoluene, whereas the growth rate of > 1.0x 10(9) s(-1) for the radical is much faster than the decay rate of (1.8+/-0.3) x 10(8) s(-1) for the precursor fluorescence in dissociation of p-methoxybenzyl alcohol. The formation of the radical depends linearly on the photolysis pulse fluence for dissociation of p-methoxytoluene and p-methoxybenzyl alcohol. The data show existence of two distinct dissociation channels. p-Methoxytoluene dissociates from thermally equilibrated levels of the S-1 state after vibrational relaxation, whereas p-methoxybenzyl alcohol dissociates from vibrationally excited levels of the S-1 state in competition with vibrational relaxation. The difference of these channels is explained on a model of electronic coupling between the precursor and product states in the geometry where the C-H and C-O bonds are stretched in a plane perpendicular to the benzene rings. For p-methoxytoluene, the S-1 state does not correlate adiabatically to the ground state of the C-H bond fission products, so intersystem crossing or internal conversion precedes dissociation. For p-methoxybenzyl alcohol, avoided crossing between the pi pi* (benzene) configuration and the np(O)sigma*(C-O) repulsive configuration results in the adiabatic potential-energy surface which evolves to the ground state of the C-O bond fission products allowing rapid dissociation. (C) 1997 American Institute of Physics.