Osmotic water permeability and solute permeability coefficient are measured using stopped-flow fluorimetry. In a vesicle that behaves as a perfect osmometer, water flux is directly proportional to imposed osmotic pressure, and solute flux is proportional to the chemical gradient across the vesicle. The flux of water and solute leads to a change in vesicle volume. This change in volume is measured by fluorescence quenching of entrapped carboxyfluorescein in the vesicle. Equations relating volume change of the vesicle to flux of water or solute from the vesicle are given to enable computation of water and solute permeability coefficients.