This International Standard specifies a method for the determination of the long polystyrene block content of raw uncompounded styrene-butadiene rubber (SBR) and raw uncompounded blends of block SBR. It is not applicable to blends of block SBR with EPDM, IIR, CSM and other halogenated rubbers. The method is applicable to raw, uncompounded latices of the above compositions and is suitable for rubbers having block polystyrene contents in the range from 5 %(m/m) to 100 %(m/m). Rubbers having block polystyrene contents of less than 5 %(m/m) may yield incorrect results unless a correction factor, based on information gained by working with such rubbers, is applied. The method is intended for use on gel-free rubbers, but it may be used on rubbers containing gel if it has been proved that the gel does not interfere.