Abstract: A new chemical mass balance technique has been developed for simultaneous mineralogical quantification and chemical characterization of soil clays. The procedure includes separation of the whole clay (<2 µm fraction) into six particle size fractions (<0.02, <0.06, <0.2, 0.02–0.06, 0.06–0.2, and 0.2–2 µm fractions), chemical analysis of the whole clay and each of the six fractions, and fitting of a non-linear chemical mass balance model to the chemical analyses. As written, the chemical mass balance model is valid only for samples containing mixtures of quartz, kaolinite, illite, and mixed-layered smectite-illite. Samples containing carbonates and free iron compounds may be analyzed using the technique if these phases are chemically removed prior to particle size fractionation. Accuracy of the new technique was tested using synthetic data and found to depend on the quality of the input data; however, clay phase quantification within three percentage points of known values was readily achieved. Precision of the technique was evaluated by independently preparing and analyzing five samples of the same soil clay. Standard deviations for clay phase percentages (w:w) in the <2 µm fraction were all less than one percent. The new technique yields accurate determinations of chemistry for the smectitic and illitic phases in mixed-layered smectite-illite, and qualitative estimates for the chemistry of 10 Å-illite. The elemental compositions of quartz and kaolinite are assumed a priori and treated as constants within the non-linear chemical mass balance model.