This International Standard specifies a method for the determination of hydroxyl numbers of polyols using NIR (near-infrared) spectroscopy. Definitions and calibration techniques are given. Procedures for selecting calibration materials and for collecting and processing data to develop NIR calibrations are outlined. Criteria for building, evaluating and validating the NIR calibration model are also described. Procedures for sample handling, data gathering and evaluation are included. It is necessary to know the hydroxyl number of polyols in order to properly formulate polyurethane systems. This International Standard is suitable for use in research, quality control, specification testing and process control.