The determination of the chemical composition of chlorites, using the ratio of structure factors for the 002 and 004 reflections, is unreliable because of the small change in the ratio with composition. By using the rapidly changing ratio F003/F001, a factor may be computed which relates theoretical structure factors to the structure factors calculated from a dif-fractogram. By determining the theoretical value of F002, the composition of the chlorite is determined. Sample thickness, detector error, time constant, and sample size must all be controlled for the successful application of this technique. In particular, the relationship between sample size and divergence slit size has a most important effect on x-ray intensities. The method is accurate to within 13 per cent of the composition of the chlorite octahedral sheets.