During June 1986, eight systems for measuring vapor phase and four measuring particulate phase concentrations of formic acid (HCOOH) and acetic acid (CH3COOH) were intercompared in central Virginia. HCOOH and CH3COOH vapors were sampled by condensate, mist, Chromosorb 103 GC resin, NaOH-coated annular denuders. NaOH impregnated quartz filters. K2CO3 and Na2CO3 impregnated cellulose filters, and Nylasorb membranes. Atmospheric aerosol was collected on Teflon and Nuclepore filters using both hi-vol and lo-vol systems to measure particulate phase concentrations. Samples were collected during 31 discrete day and night intervals of 0.5--2 hour duration over a 4-day period. Performance of the mist chamber and K2CO3 impregnated filter techniques were also evaluated using zero air and ambient air spiked with HCOOHg, CH3COOHg, and formaldehyde (CH2Og) from permeation sources. Results of this intercomparison show significant systematic and episodic artifacts among many currently deployed measurement systems for HCOOHg and CH3COOHg. The spiking experiments revealed no significant interferences for the mist chamber technique and results generated by the mist chamber and denuder techniques were statistically indistinguishable. The condensate technique showed general agreement with the mist chamber and denuder methods, but episodic bias between these systems was inferred from large and significant differences observed during the first day of sampling. Nylasorb membranes are unacceptable for collecting carboxylic acid vapors as they did not retain HCOOHg and CH3COOHg quantitatively. Strong base impregnated filter and GC resin sampling techniques are prone to large positive interferences apparently resulting, in part, from reactions involving CH2Og to generate HCOOH and CH3COOH subsequent to collection. Significant bias presumably associated with differences in postcollection handling was observed for particulate phase measurements by participating groups. Analytical bias did not contribute significantly to differences in vapor and particulate phase measurements. ¿ American Geophysical Union 1989