ISO 4589-2:2017 specifies methods for determining the minimum volume fraction of oxygen, in admixture with nitrogen, that will support combustion of small vertical test specimens under specified test conditions. The results are defined as oxygen index (OI) values. Methods are provided for testing materials that are self-supporting in the form of vertical bars or sheets up to 10,5 mm thick. These methods are suitable for solid, laminated or cellular materials characterized by an apparent density 100 kg/m3 or greater. The methods might also be applicable to some cellular materials having an apparent density of less than 100 kg/m3. A method is provided for testing flexible sheets or film materials while supported vertically. For comparative purposes, a procedure is provided for determining whether or not the OI of a material lies above some specified minimum value.