Spectrophotometric analysis affords the most convenient means for determining the proportion of methylene blue and trimethyl thionin (azure B) present in a mixture of these two dyes. The method proposed depends upon the determination of an “absorption ratio.” A suitable ratio for the purpose is that of the extinction coefficient at 640 mμ to that at 670 mμ. On account of the difference in absorption maxima of the two dyes, this ratio increases as the percentage of methylene blue decreases. The ratio value for eleven different mixtures is given and a graph is plotted from this data by means of which the proportions of the two dyes present in any mixture can be calculated from the absorption ratio determined as specified.