AbstractChitosan was used as a sizing promoter to improve the sizing performance of ASA emulsion stabilized by montmorillonite. It was found that chitosan with low molecular weight significantly improved the sizing performance of ASA emulsion without inducing the flocculation of the ASA droplets when the amount was less than 0.125% based on ASA. Confocal fluorescence microscopy revealed the formation of chitosan shell around the ASA droplet. Adding ammonia into the emulsion induced desorption of chitosan from the ASA droplet surface and the ammonolysis of ASA. The former reduced the ASA emulsion stability, while the latter resulted in the secondary emulsification of ASA-water system. Meanwhile, the presence of ammonia was demonstrated to be detrimental to the sizing performance of the ASA emulsion. The hydrolysis resistance of the ASA emulsion was higher when chitosan was present.