A tandem mass spectrometer modified for aircraft applications was used to measure gas-phase formic and acetic acid mixing ratios over the western North Atlantic during late summer 1992. The sensitive, specific, and essentially real-time measurements provided by the mass spectrometer allow a true spatial evaluation of these compounds in the study area. Formic and acetic acid mixing ratios showed substantial vertical variation, varying by factors of up to 13 and 6, respectively, within 2-km profiles extending from the boundary layer into the free troposphere. Substantial horizontal variation was also observed at constant altitudes within both the boundary layer and free troposphere. Mixing ratios of the two acids were correlated (r2>0.70) throughout the study region. ¿ American Geophysical Union 1995