Results of narrowband hole burning experiments and picosecond photon echo experiments on pentacene in benzoic acid are presented. On burning, several new discrete pentacene absorptions (antiholes) are created. These spontaneously revert to the unburned form at low temperatures, but may also be induced to revert by optical excitation. Deuteration of the host acidic protons markedly changes the pentacene fluorescence lifetime and hole burning and recovery behavior, indicating the strong interaction of pentacene and the benzoic acid dimer hydrogen bonds. Hole burning and antihole formation are attributed to hydrogen bond tautomerism of benzoic acid dimers near excited pentacenes. Two mechanisms for this tautomerization are suggested. The homogeneous linewidth of the pentacene S0→S1 transition is measured by both photon echo and hole burning experiments. A preliminary study of the homogeneous linewidth measured by hole burning indicates the transition broadens with an 11 cm-1 thermal activation energy, which differs from measurements made by other methods. © 1982 American Institute of Physics.