X-ray microanalysis have been made on a suite of nine oilwell cement clinkers. Elemental data were obtained on alite, belite, and ferrite phases and, for two clinkers only, also on aluminate. The alite and belite compositions are broadly similar to those previously reported for other portland cements. Guest ion concentrations suggest that several charge-balancing substitutions occur. bulk MgO and SO3 levels determine the Mg and S contents of both phases, but for Fe and Al the substitution levels are not strongly correlated with bulk composition. For ferrites the data are generally consistent with the results of Bergstrom et al. (Adv Cem Res 1992, 4, 141-147): the Mg content varies widely, is controlled by the bulk MgO, and is coupled with Si in a charge-balancing substitution for Fe. Estimates of the total phase assembly using the directly determined mineral compositions are compared with the predictions of a modified Bogue calculation (similar to that of Taylor, Adv Cem Res, 1989, 2, 73-77). The agreement is variable and some refinements to the method of calculation are indicated.