This International Standard specifies two methods for the measurement of the hydroxyl number of polyols used as polyurethane raw materials. It is necessary to know the hydroxyl content of polyols to properly formulate polyurethane systems. Method A is primarily applicable to readily esterified polyether polyols. It is also recommended for polyols which have significant steric hindrance, such as those based on sugars. Method B is recommended for polyether polyols, polymer polyols and amine-initiated polyols, but may give low results for sterically hindered polyols. Other polyols may be analysed by these methods if precautions are taken to verify applicability. These methods can be used for research and for quality control and specification purposes.