A new method for the analysis of formic acid was developed using gas chromatography-electron impact ionization mass spectrometry in the selected ion monitoring mode and solid-phase microextraction. Using this method with [13C]formic acid as an internal standard, the peak area ratio of [12C]formic acid/[13C]formic acid was not affected by differing methanol or sulfuric acid concentrations during the esterification and fiber adsorbing step. In comparison, the peak area ratio of formic acid/acetonitrile as detected by conventional GC with flame ionization detection was greatly affected by methanol or sulfuric acid concentrations. The formic acid calibration curve of our method showed excellent linearity over the range 5 to 200 microM. The within- and between-run assay relative standard deviations for the formic acid concentration were all less than 1.70%.