Organic acids are important contributors to ambient acidity, in both gas and particulate phase. Particulate phase organic acids represent an important fraction of organic particulate matter. This paper presents the results of a field study conducted in Philadelphia, Pennsylvania, during the summer of 1992. The purpose of this study was to measure the concentrations of gas and particulate phase organic acids. Formic acid was found to be the most abundant gas phase organic acid, with acetic and propionic acids detected at lower concentrations. Dicarboxylic acids and even-carbon monocarboxylic acids were found to be present at relatively high concentrations in the particulate phase; odd-carbon monocarboxylic acids were present at much smaller concentrations. This paper discusses the measurement and analytical techniques used in this study. A subsequent paper will address the data analysis and discuss the chemistry and origins of organic acids. ¿ American Geophysical Union 1996