ISO 6271-2:2004 specifies a spectrophotometric method for estimating the colour, in Pt-Co units, of clear liquids. It is applicable to clear liquids having a colour characteristic similar to those of the reference platinum-cobalt scale specified in ISO 6271-1, which describes a visual method. The method described provides a more precise way of measuring Pt-Co colour than that described in ISO 6271-1.