In the previous paper we discussed the sampling and analytical techniques used to measure the concentrations of gas and particulate phase organic acids. This paper presents the results of a field study conducted in Philadelphia, Pennsylvania, during the summer of 1992, to investigate the chemistry and origins of gas and particulate phase organic acids. The concentrations of formic acid and most of the particulate phase organic acids were found to be significantly higher during the day than at night. Organic acids constituted 5.5% of the particulate fine mass (PM2.5); sulfates, ammonium, and elemental carbon were larger components of particulate fine mass, constituting 40, 15, and 9%, respectively. Dicarboxylic acids and even-carbon monocarboxylic acids were found to account for a large fraction of particulate weak acidity; odd-carbon monocarboxylic acids accounted for a very small fraction. The pronounced even-carbon preference of the monocarboxylic acid distribution suggests a biogenic origin. To investigate the origin of gas and particulate phase organic acids with regard to other species measured as part of the same field study, specific rotation factor analysis (SRFA) was applied to the daytime and nighttime concentrations of several species over the study period. During the daytime, four sources (transport, local combustion, biogenic, and photochemical) accounted for 92--99% of the particulate phase and 60--75% of the gas phase organic acids. At night, 68--87% of gas and particulate organic acids were contributed by four significant sources (transport, daytime concentration related, local combustion, and biogenic). ¿ American Geophysical Union 1996