A method of sizing a hydrophobic yarn for weaving comprising the steps of applying thereto an acid pH latex of a free radical initiated vinyl polymer and drying the yarn. The polymer has a weight average molecular weight of from about 300,000 to about 2,500,000, and in dried film form is hydrophobic, not redispersible in a solution at a low pH and redispersible in an alkaline aqueous medium. The polymer is prepared from monomers comprising about 8% to about 14%, by weight ethylenically unsaturated acid. The average solubility parameter of the monomers in the polymer is between about 9.0 and about 9.5; the calculated polymer T.sub.g is between about -20.degree. C. and 20.degree. C. Preferred yarns are nylon and polyester.