The authors used gas chromatographic headspace analysis to study the sodium chloride concentration dependence of the partitioning of acetonitrile, ethanol, n-propanol, and t-butanol from water and plasma to headspace vapor. Increasing the sodium chloride concentration caused logarithmic increases in the partitioning. At 25 degrees C the slopes (log10[peak height]/mol sodium/L) obtained with the use of water or plasma were as follows: acetonitrile, 0.064 (0.059); ethanol, 0.126 (0.125); n-propanol, 0.152 (0.149); and t-butanol, 0.200 (0.183). Differences in water content between the two liquids may contribute to the small differences in the regression data. More importantly, saturation with sodium chloride at 25 degrees C produced solutions with different sodium molarities: 5.2 mol/L for water and 4.8 mol/L for plasma. This difference in salt concentration at saturation and the volatile dependent slopes can account for a large part of the error in plasma ethanol concentrations when measured with the use of aqueous external standardization and internal standardization with any of the other volatiles. Deproteinization of the plasma abolished the liquid phase-dependent differences in saturated salt concentration and partitioning.