The osmotic coefficients of 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium tetrafluoroborate in water, 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate in acetonitrile were determined by the vapor pressure osmometry method at 318.15 K. The aqueous solutions of NaCl up to 2.5 mol kg(-1) and LiBr in acetonitrile up to 0.8 mol kg(-1) were used as the reference solutions with precise osmotic coefficients calculated from vapor pressure data. Obtained experimental osmotic coefficients are reliably correlated by Archer extension of Pitzer model. The parameters for these equations, along with the corresponding standard deviations are presented for these systems. The obtained parameters were used to calculate the mean molal activity coefficients. (c) 2007 Elsevier B.V. All rights reserved.