An emulsion-polymerized addition copolymer formed from a monomer mixture comprising ethylenically unsaturated monomers, including from more than 4 to 15 wt % based on the weight of said mixture of at least one ethylenically unsaturated carboxylic acid functional monomer, and from 0.05 to 5 wt % based on the weight of said mixture of at least one polymerizable surfactant monomer comprising hydrophobic and hydrophilic functional groups, wherein said hydrophobic functional group comprises a polymerizable group within it, is useful as a binder in an ink jet ink having improved print quality.