The molality dependence of isopiestic osmotic coefficients of aqueous guanidinium sulphate, Gn(2)SO(4) has been reported at 298.15 and analysed by the Fitter equations. The Fitter coefficients obtained from the analysis of osmotic coefficients are employed to calculate the activity coefficients. The viscosity and surface tension of aqueous guanidinium chloride (GnCl), bromide (GnBr), acetate (CH(3)COOGn), perchlorate (GnClO(4)) and sulphate (Gn(2)SO(4)) have also been measured at 298.15 K. The order in which these salts increase the viscosity and surface tension of water is: Gn(2)SO(4) > CH(3)COOGn > GnCl > GnBr > GnClO(4). The effect of Gn(2)SO(4) and CH(3)COOGn on the viscosity and surface tension is stronger than that of other guanidinium salts.