A method has been derived to estimate both the bulk composition and the distribution of Mg and Mn and of Mg and Ca atoms between M1 and M2 octahedral sites in olivines of the forsterite-tephroite and forsterite-monticellite series, making use of the regression equations, a = 0.932r M1 + 0.236r M2 + 3.918, b = 0.505r M1 + 3.211r M2 + 7.535, determined by Lumpkin and Ribbe (1983). The strong dependence of a on r M1 , the radius of the M1 cation, and of b on r M2 is exploited on an a-b plot contoured for composition, 100 Mg/[Mg + (Mn or Ca)], and for the Mn (or Ca) content of the M2 site. Predictions of bulk composition from a-b are within + or -1.5% Fo (on the average), and of site occupancies are usually better than + or -0.05 Mn or Ca. The d-spacings for the 130 and 131 peaks in X-ray powder patterns of olivines are very sensitive to Mg-Mn and Mg-Ca order-disorder, but d 112 was found to be insensitive (to within one part in 2500) to cation distribution in M1 and M2. Regression equations to estimate compositions are 100 Mg/[Mg + Mn + minor (Fe,Zn,Ca)] = 2484.87 - 970.42d 112 , and 100 Fo/(Fo + Mo) = 1946.25 - 752.14d 112 ; precision is estimated at better than + or -1.5 mole% Fo.--Modified journal abstract.