An “operational” definition of osmotic pressure is presented and the difference between this definition and the thermodynamic definition of osmotic pressure is pointed out. By the use of a negative pressure osmometer a formula is developed to express the values of “osmotic pressures” when the osmometer membrane is permeable to both solvent and solute. A membrane factorBis introduced to account for the efflux of solute from the osmometer.Bis evaluated in terms of the effective solute diffusion coefficient,D.