The Ostwald solubility coefficient, L of 17 volatile organic compounds (VOCs) from the gas phase into water and dilute aqueous ammonia solutions was determined by the equilibrium partitioning in closed system-solid phase micro extraction (EPICS-SPME) method at 303 K and at 0-2.5 mol dm(-3) ammonia concentrations. Ammonia increased the solubility of all VOCs nearly linearly, but to a different extent. The difference in the solubility values in aqueous ammonia solutions (L-mix) compared to pure water (L) is explained on the basis of a Linear Solvation Energy Relationship 7rH - aH Z H + (LSER) equation made applicable for solvent mixtures, logL(mix) - logL = x((s(NH3) - s(H2O)) pi(H)(2) + (a(NH3) - a(H2O)) Sigma,alpha(H)(2) + (b(NH3) - b(H2O))Sigma,beta(H)(2) + (v(NH3) - v(H2O))V-x). s(NH3) - s(H2O), a(NH3) - a(H2O), v(NH3) - v(H2O) are the differences of solvent parameters, x is the mole fraction, pi(H)(2) is the solute dipolarity-polarizability, Sigma,alpha(H)(2) is the effective hydrogen bond acidity of the solute, Sigma,beta(H)(2) is the effective hydrogen bond basicity of the solute and V-x, the McGowan characteristic volume. The most significant term was v, the phase hydrophobicity. The solubility behavior was explained by the change in structure of the aqueous solution: the presence of ammonia reduces the cavity effect.