A composition containing: modified polyvinyl alcohol; and unsaturated monocarboxylic acid or a salt thereof, wherein the modified polyvinyl alcohol has a viscosity-average degree of polymerization of from 400 to 3500, has a degree of saponification of from 68 mol % to 99.9 mol %, and contains from 0.01 mol % to 1.50 mol % of an acryloyl group or a methacryloyl group in a side chain, the unsaturated monocarboxylic acid or a salt thereof is at least one selected from the group consisting of acrylic acid, methacrylic acid, sodium acrylate, and sodium methacrylate, and in the composition, a mass ratio of the modified polyvinyl alcohol/the unsaturated monocarboxylic acid or a salt thereof is from 82/18 to 99.9/0.1. The composition is thus provided that is excellent in water solubility and storage stability and polymerization stability of a vinyl compound and contains the modified polyvinyl alcohol.