Abstract: A process for decreasing the concentration of phenothiazine from a solution of acrylic acid by contacting the acrylic acid solution with a hydrophobic clay, such as an organophilic clay; or by protonating the phenothiazine, oxidizing the phenothiazine, and contacting the protonated, oxidized phenothiazine with a hydrophilic clay, such as an acid-activated smectite clay or sodium bentonite clay to sorb (absorb and/or adsorb) the phenothiazine into or onto the clay. Once sorbed onto the smectite clay, the phenothiazine easily can be removed, together with the clay, from the solution of acrylic acid, e.g., by filtration.