X‐ray diffraction patterns of aqueous solutions of the oxazine dye, Nile blue sulfate, yield direct evidence of a varying molecular association as a function of concentration, which was previously indicated by Cohen's potentiometric and spectrophotometric data. Solutions of the order of millionth molar approach obedience to the laws of dilute solutions; in moderate concentrations of the order of 5×10−4molar the dye molecules associate to micellar structures; and in concentrated solutions the association proceeds to the stage of precipitation. The largest change in the value of the identity spacingdcorresponding to the inner edge of the principal diffraction halo is found to begin with about 10−4molar solutions. The attempt is then made to measure pore sizes in cotton fibers from the relative absorbability of the dye from its solutions over such a range of molecular and aggregate dimensions. Nile blue sulfate and methylene blue solutions before and after addition of absorbent cotton were subjected to spectrophotometric analysis. Again it is indicated in the case of the former that there is a large increase in dye absorption by the cotton corresponding to the change with increasing dilution from micellar to molecular dimensions which are compatible with predominating cotton pore sizes.