The present invention relates to a composition comprising an aqueous dispersion of multistage polymer particles with a first phase comprising structural units of a carboxylic acid monomer and a nonionic ethylenically unsaturated monomer; and a second phase with a calculated Tg of less than 50° C. and comprising a mixture of polymers which together comprise structural units of styrene or methyl methacrylate or both; structural units of one or of n-butyl acrylate, 2-ethylhexyl acrylate, ethyl acrylate, or n-butyl methacrylate; and structural units of a carboxylic acid monomer. The composition is useful as an open time additive in coatings formulations.