The S-1/S-2 splitting of the m-cyanophenol dimer, (mCP)(2) and the delocalization of its excitonically coupled S-1/S-2 states are investigated by mass-selective two-color resonant two-photon ionization and dispersed fluorescence spectroscopy, complemented by a theoretical vibronic coupling analysis based on correlated ab initio calculations at the approximate coupled cluster CC2 and SCS-CC2 levels. The calculations predict three close-lying ground-state minima of (mCP)(2): The lowest is slightly Z-shaped (C-i-symmetric); the second-lowest is <5 cm(-1) higher and planar (C-2h). The vibrational ground state is probably delocalized over both minima. The S-0 -> S-1 transition of (mCP)(2) is electric-dipole allowed (A(g) -> Au), while the S-0 -> S-2 transition is forbidden (A(g) -> A(g)). Breaking the inversion symmetry by C-12/C-13- or H/D-substitution renders the S-0 -> S-2 transition partially allowed; the excitonic contribution to the S-1/S-2 splitting is Delta(exc) = 7.3 cm(-1). Additional isotope-dependent contributions arise from the changes of the m-cyanophenol zero-point vibrational energy upon electronic excitation, which are Delta(iso)(C-12/C-13) = 3.3 cm(-1) and Delta(iso)(H/D) = 6.8 cm(-1). Only partial localization of the exciton occurs in the C-12/C-13 and H/D substituted heterodimers. The SCS-CC2 calculated excitonic splitting is Delta(el) = 179 cm(-1); when multiplying this with the vibronic quenching factor Gamma(exp)(vibron) = 0.043, we obtain an exciton splitting Delta(exp)(vibron) = 7.7 cm(-1), which agrees very well with the experimental Delta(exc) = 7.3 cm(-1). The semiclassical exciton hopping times range from 3.2 ps in (mCP)(2) to 5.7 ps in the heterodimer (mCP-h)(mCP-d). A multimode vibronic coupling analysis is performed encompassing all the vibronic levels of the coupled S-1/S-2 states from the v = 0 level to 600 cm(-1) above. Both linear and quadratic vibronic coupling schemes were investigated to simulate the S-0 -> S-1/S-2 vibronic spectra; those calculated with the latter scheme agree better with experiment.