Results are presented of measurements of the concentration of gas phase H2SO4 and methane sulfonic acid (MSA) performed aboard the NASA P3-b aircraft during the Pacific Exploratory Mission (PEM) Tropics study using the selected ion chemical ionization mass spectrometry (SICIMS) technique. During a nighttime portion of one flight the <H2SO4> was found to increase with decreasing relative humidity (RH). When compared to laboratory measurements of H2SO4 vapor pressure as a function of RH and particle neutralization (NH4+ and SO42- ionic composition) and model predictions using a liquid drop hydrate model, these measurements indicate that the particles from which the H2SO4 is evaporating are relatively unneutralized, a result which is in good agreement with filter measurements. Overall, the same increase in the gas phase with decreasing RH or decreasing <NH4+> (obtained from filter measurements) was also observed, indicating a high volatility of MSA at low RH values or particle neutralization. When gas phase MSA values are compared to methane sulfonate (MS) values obtained from filter measurements, it was found that MSA was totally volatilized at low RH values, while MSA resides mainly in the particulate form at high RH values. Combining the gas phase and filter measurements, the boundary layer MS/(MS+SO42-) ratio showed a distinct increase with decreasing temperature and suggests that little or no MS or MSA is produced in the boundary layer at temperatures above 300 K. ¿ 1999 American Geophysical Union