Radiative dynamics of the hemicyanine dye N-(3-sulfopropyl-4-(p-dioctylaminostyryl)pyridinium (I) have been studied in primary alcohols and in Langmuir-Blodgett (LB) monolayer films by using time-resolved laser-induced fluorescence spectroscopy. In solution, the deactivation of the optically excited state is dominated by a radiationless transition via an activated barrier crossing to the twisted intramolecular charge transfer (TICT) state, and the decay rate decreases substantially with increasing n in the series CH3(CH2)(n)OH. Observed biexponential decays are consistent with the three-level scheme describing the horizontal transition on the excited-state potential surface. In monolayer films the decay dynamics are similar whether the initial excitation occurs into the monomer or aggregate bands. The kinetics are interpreted in terms of diffusion of excitons in the aggregate with eventual energy transfer to the monomer. A Monte Carlo simulation was used to characterize the motion of the excitons within aggregates and to correlate the aggregate size with exciton lifetime. The finite size of the aggregate in monolayer films was estimated under two limiting cases : one in which exciton transport on the aggregate is the rate-limiting step in emission and another in which aggregate-monomer excitation transfer limits the radiative rate. Comparison of experimental and simulated results yielded an estimate of the size of an H-aggregate in monolayers of I to be ca. 45 molecules in the first case and 50 molecules in the second. A comprehensive kinetic scheme is presented that unifies both the solution and monolayer film behavior observed in these studies.