We calculate isotope shifts and perform a normal-coordinate analysis to explain the observed systematic variation with isotopic composition of the intersystem crossing for deuterated pentacene in p-terphenyl. The tuning of a near resonance of the first excited singlet state S-1 and vibrational levels of T-3 by S-1 <-- T-3 isotope shifts is analyzed. The physics of isotope shifts is discussed, and an explanation is given for the phenomenological sum rule dictating additivity of isotope shifts upon multiple substitution. We discuss mechanisms through which the host matrix interactions influence isotope shifts and increase inhomogeneous broadening.