Abstract: A novel and improved method for polymerizing a vinyl monomer in a polymerization reactor is provided. The inner walls of the polymerization reactor and other surfaces coming into contact with the monomer are coated with an aqueous liquid containing at least one alkali metal or ammonium salt of an anionic dye of the sulfonic carboxylic acid form and having a pH value adjusted not to exceed 7 by aid of a pH adjusting agent, and the thus coated surfaces are dried with heat before the polymerization is conducted. The method serves to greatly reduce the deposition of polymer scale on those surfaces and, consequently, improve the productivity as well as the quality of polymer product.