The polymer latexes of poly(MMA-AA) were synthesized using two techniques: soapless seeded emulsion polymerization, and soapless emulsion copolymerization. Then a series of rigid polymer films containing a COOH group were prepared from the synthesized latex. The humidity-sensitive properties of the polymer films were investigated. Our studies confirmed that there exists both an optimum ratio of hydrophilic to hydrophobic monomers and initial structure of the latex particle that provides the humidity-sensitive polymer film with excellent water resistivity and sensitivity to humidity. In addition, little hysteresis and quick response were observed.