The objective of this work is to establish a colorimetric method for the quantitative determination of montmorillonite. It is based on reaction of this clay mineral with benzidine, because the colouring complex follows the law of Lambert Beer in the interval from 7,35 to 36,75 ug of montmorillonite/ ml. Various concentrations of this mineral were treated with 1,0 ml of an aqueous solution of benzidine, and the absorbance of the complex was readen after 48 hours with a filter number 60 in a "Klett-Summerson" colorimeter. With these data it was made an equacion reporting the concentration against the absorbance, what is: X - 5L - 14,7 (L is absorbance and X is ug of montmorillonite). It was also tested the influence of other minerals as quartz and kaollinite. The conclusion was that they increase the absorbance. In this test two new equacions were obtained: X = 4,17L - 30,04 for the misture of montmorillonite, quartz and kaollinite and X = 4,76L - 60,33 for the misture of montmorillonite and quartz. In the misture where the three minerals are present the increases are higher than when only quartz was present.