A computer model was used to predict perpendicular flame quenching distances for methanol-air mixtures at a variety of conditions. A detailed kinetic reaction mechanism has allowed the study of aldehyde formation. Aldehydes were found to increase an order of magnitude over bulk gas concentrations as the flame quenches but this concentration is insufficient to account for measured exhaust aldehydes.A flow reactor study showed additional aldehydes being formed from the unburned quench layer when it mixes with the hot exhaust gases during the expansion and exhaust strokes. Quenching distance decreases with increasing pressure and increasing wall temperature, and increases with lean and very rich equivalence ratios and increasing exhaust gas recirculation. Water addition shows only a slight increase in quenching distance for up to 30% of the fuel by volume.