In this work, the osmotic properties of the binary mixtures of 1-butyl-1-methylpyrrolidinium iodide ([BmPr] [I]) with isopropanol and n-butanol were studied by using vapor pressure osmometry (VPO) technique at 323.15 K at atmospheric pressure. It was found that the values of osmotic coefficient, mean molal activity coefficient, and vapor pressure decrease with increasing molality. We observed that the change in vapor pressure of ionic liquid in n-butanol is smaller compared to ionic liquid in isopropanol. The excess Gibbs free energy of ionic liquid in isopropanol has a higher value as compared to an ionic liquid + n-butanol mixture. The binary mixture of ionic liquid with alcohols shows noticeable deviation from the Debye-Huckel limiting law. We have used the pseudophase separation model on the experimental osmotic coefficient value to estimate the critical aggregation concentration of ionic liquid in alcohols. We have showed the structural effect of the solvent on the preceding thermodynamic parameters and critical aggregation concentration value of the ionic liquid in alcohols.