Monte Carlo simulations of the pyridinium N-phenolate dye "betaine-30" in 12 solvents (20 solvent representations) were performed in oi-der to explore the molecular basis of the E-T(30) scale of solvent polarity. Ab initio (HF/6-31G*) and senniempirical (AMI ana INDO/S) electronic structure calculations were used to determine the geometry and charge distribution of betaine-30 in its S-0 and S-1 states. The solvent effect on the betaine absorption spectrum was assumed to derive from electrostatic interactions between the effective charge distributions of solvent molecules and the charge shift brought about by the S-0 --> S-1 transition. Two models for this charge shift, one obtained from INDO/S calculations and the other an idealized two-site model, were used for the spectral calculations. Good agreement between simulated and observed Delta E-T shifts (E-T(30) values measured relative to the nonpolar standard tetramethylsilane) was found for both charge-shift models. In water and other hydroxylic solvents, the O atom of the betaine solute was observed to form moderately strong hydrogen bonds to between one and two solvent molecules. The contribution of these specifically coordinated molecules to the Delta E-T shift was found to be large, (30-60%) and comparable to experimental estimates. Additional simulations of acetonitrile and methanol in equilibrium with the S1 state of betaine-30 were used to determine reorganization energies in these solvents and to decide the extent to which the solvent response to the S-0 <----> S-1 transition conforms to linear response predictions. In both solvents, the spectral distributions observed in The S-0 state simulations were similar to 15% narrower than those in the S-1 simulations, indicating only a relatively small departure from linear behavior. Reorganization energies were also estimated for a number of other solvents and compared to values reported in previous experimental and theoretical studies.