Stilbazolium dyes, which form H-aggregates upon condensation at the air-water interface. typically display nonlinear optical (NLO) activity far below what would be predicted on the basis of their large intrinsic molecular hyperpolarizability, beta. Three separate hypothesis have been put forward to explain this observation : structural inversion to produce a centrosymmetric head-to-tail structure. local field effects, which act to reduce the effective magnitude of the nonlinear driving field, and changes in electronic structure, which shift the electronic resonance frequencies for the aggregates. Investigation of the linear and nonlinear spectroscopy of the dye 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide were performed on a series of samples in which the aggregation state of the structure was changed systematically, but without changing the composition of the film. These experiments shaw that, for this particular dye. changes in electronic structure upon aggregation comprise the most important factor in determining the reduction in second harmonic generation (SHG) efficiency of the aggregate.