Single pentacene molecules containing C-13 or H-1 in a pentacene-d(14) doped p-terphenyl crystal have been studied by fluorescence autocorrelation. The triplet dynamics has been analyzed and a systematic dependence of the S-1 --> T-1 intersystem crossing rate on isotopic composition was found. This variation is discussed in terms of a modulation of the near resonance of the first excited singlet state S-1 and vibrational levels of an intermediating triplet state T-3 which results from the distinct isotope dependence of the zero-point energy of both electronic states.