The Bogue calculation gives low results for alite and inaccurate ones for the other phases in Portland cement clinkers, largely because the compositions of the phases differ markedly from those of the unsubstituted compounds which it assumes to be present. For a wide range of rapidly cooled clinkers produced in modern plants, the compositions of the phases are sufficiently constant to justify the use, with minor modifications, of average values based on X-ray microanalyses, instead of those of the unsubstituted compounds. On this basis, a procedure is described which, like the Bogue calculation, is essentially a solution of four linear simultaneous equations. A method for estimating the contents of sulphate components from the bulk composition is also described. These procedures together yield a mass balance table showing how the oxide components, both major and minor, are distributed among the major and minor phases. Limitations, modifications, extensions to special types of Portland cement and a test of the procedure for the major phases are described.