Intrinsic activation energies E-0(#) for different adiabatic photoreactions have been derived by combining pressure and temperature dependent time-resolved fluorescence data. This procedure allows the division of the observable activation energy E-obs(#) into a diffusive barrier E-d(#) induced by the solvent viscosity and an intrinsic part E-0(#), due to the intramolecular reaction coordinate, and furthermore, the procedure corrects for polarity effects on the rate constant. When applied to the kinetics of charge transfer state formation in DMABN (N,N-dimethylaminobenzonitrile), it establishes the barrierless nature of the excited state hypersurface in the investigated solute/solvent systems. This case of a negligible intrinsic activation energy is characterized by nonexponential kinetics whereas a comparable case with barrier, the excimer formation in DIPHANT, an intramolecular 9-phenylanthracene excimer molecule, shows exponential decays. For this system in polydimethylsiloxane (S1000), an intrinsic thermal barrier of 17 kJ/mol was determined.