The composite latex particles of poly(methyl methacrylate)-poly(methacrylic acid) [poly(MMA-MAA)] were synthesized through either soapless seeded emulsion polymerization or a soapless emulsion copolymerization technique. The reaction kinetics, morphology, and size of latex particles, composition, glass transition temperature (T-g), and molecular weight of polymer products were studied under different experimental conditions. Moreover, this work also focused on the humidity sensitive properties of the polymer films fabricated by melting under the temperature of 200 degrees C and followed by chemical modification with aqueous solution of NaOH. It is confirmed that there exists both an optimum ratio of hydrophilic to hydrophobic monomers and the initial structure of the latex particle to provide the humidity-sensitive polyelectrolyte film with excellent water resistivity and good sensitivity to humidity. Besides, little hysteresis and quick response were observed.