The excitonic S-1/S-2 state splitting and the localization/delocalization of the S-1 and S-2 electronic states are investigated in the benzonitrile dimer (BN)(2) and its C-13 and d(5) isotopomers by mass-resolved two-color resonant two-photon ionization spectroscopy in a supersonic jet, complemented by calculations. The doubly hydrogen-bonded (BN-h(5))(2) and (BN-d(5))(2) dimers are C(2)h symmetric with equivalent BN moieties. Only the S0 -> S2 electronic origin is observed, while the S-0 ? S-1 excitonic component is electric-dipole forbidden. A single C-12/C-13 or 5-fold h(5)/d(5) isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 h(5)(13)C), (BN)(2)-(h(5) d(5)), and (BN)(2)-(h(5) h(5)(13)C) exhibit both S-0 -> S1 and S0 ? S2 origins. Isotope-dependent contributions ?iso to the excitonic splittings arise from the changes of the BN monomer zero-point vibrational energies; these range from ?iso(C-12/C-13) = 3.3 cm(-1) to ?iso(h(5)/d(5)) = 155.6 cm1. The analysis of the experimental S-1/S2 splittings of six different isotopomeric dimers yields the S-1/S-2 exciton splitting (exc) = 2.1 +/- 0.1 cm1. Since ?iso(h(5)/d(5)) ?exc and iso(C-12/C-13) > ?exc, complete and near-complete exciton localization occurs upon C-12/13C and h(5)/d5 substitutions, respectively, as diagnosed by the relative S-0 -> S-1 and S-0 -> S-2 origin band intensities. The S-1/S-2 electronic energy gap of (BN)(2) calculated by the spin-component scaled approximate second-order coupled-cluster (SCS-CC2) method is ?elcalc = 10 cm1. This electronic splitting is reduced by the vibronic quenching factor G. The vibronically quenched exciton splitting elcalcG = ?vibroncalc = 2.13 cm1 is in excellent agreement with the observed splitting ?exc = 2.1 cm1. The excitonic splittings can be converted to semiclassical exciton hopping times; the shortest hopping time is 8 ps for the homodimer (BN-h(5))(2), the longest is 600 ps for the (BN)(2)(h(5) d(5)) heterodimer.