The concentrations of gas phase formic acid (HCOOH) and acetic acid (CH3COOH) were measured at the Mauna Loa, Observatory, Hawaii during the first three weeks (May 1--21, 1988) of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX). Daytime values were determined mostly during upslope conditions and represent marine boundary layer air modified by contact with the island of Hawaii. Mean values were 452 and 374 pptv for HCOOH and CH3COOH, respectively. Nighttime values were determined mostly during downslope conditions and represent air in the free troposphere near 3400 m above sea level. Mean values were 63 and 94 pptv for HCOOH and CH3COOH, respectively. Aerosol formate and acetate concentration were one to two orders of magnitude less than the gas phase concentration and ranged from less than a few pptv to nearly 50 pptv. ¿ American Geophysical Union 1992