Vapor phase osmometry (VPO), also known as vapor-pressure osmometry, is an experimental technique for the determination of a polymer's number average molecular weight, M. It works by taking advantage of the decrease in vapor pressure that occurs when solutes are added to pure solvent. This technique can be used for polymers with a molecular weight of up to 20,000 though accuracy is best for those below 10,000. Although membrane osmometry is also based on the measurement of colligative properties, it has a lower bound of 25,000 for sample molecular weight that can be measured owing to problems with membrane permeation.