The study of adsorption of cationic dye (oxazine-1) on a surface of cellulose acetate microfiltration membrane VS-0.45 is carried out. It is determined that the limiting adsorption value is equal to 2,2 -10~ⁿ mole/cm² and the area of one adsorbate molecule is equal to 7,5 nm², what is significantly bigger than the own size of a dye molecule. The comparison of values of electrokinetic potentials for the initial and modified by the dye membranes in NaCl solutions is shown, that adsorption of dye decreases the absolute ( value without the change of the (^-potential sign.