The quantum yield of triplet formation (phi(T)) for two nonfluorescent molecules of 9-acetylanthracene and 9-benzoylanthracene was determined with a time-resolved thermal lensing method (TRTL). The value of phi(T) for 9-benzoylanthracene was 0.19 in cyclohexane, indicating that a fast internal conversion process with a rate constant of 4.0 x 10(10) s(-1) exists in the deactivation from the first excited singlet state (S-1).