Nonexponential decay of the 2(1)A(g)(S-1) --> 1(1)A(g)(S-0) fluorescence observed for I,6-diphenyl-1,3,5-hexatriene (DPH) static vapor in the presence of perfluorohexane used as a buffer gas is shown to be induced by intermolecular collision. The pressure and excitation energy dependence of the decay profiles are interpreted in terms of a simple two-level scheme for the 2(1)A(g) manifold and are analyzed to evaluate the collision efficiency on the vibrational relaxation, which is found to depend on the excitation energy.