Excited state lifetimes of hypoxanthine and four methylxanthine compounds (paraxanthine, theophylline, theobromine, and caffeine) were studied by femtosecond transient absorption spectroscopy in aqueous and acetonitrile solution. Transient absorption signals recorded at visible and UV probe wavelengths reveal that internal conversion from the photoexcited state to the electronic ground state takes place in water on the hundreds of femtoseconds time scale. Excited-state relaxation occurs approximately threefold more slowly in acetonitrile solution than in water. Structural considerations suggest that the deactivating conical intersection for the methylxanthines differs from that responsible for nonradiative decay by hypoxanthine, adenine, and guanine. All compounds studied have ultrashort excited state lifetimes similar to those of adenine and guanine, suggesting that these xanthine derivatives could have been photostable building blocks in prebiotic environments exposed to intense UV radiation.