The industrially significant chain extender epichlorohydrin (chloromethyloxirane) can be quantified on-line at the low parts per million level in aqueous solution using membrane introduction mass spectrometry (MIMS). Solid membranes composed of microporous poly(tetrafluoroethylene) (PTFE) and nonporous silicone were used to establish this methodology using an ion trap mass spectrometer; both gave similar performance, even though they are based on different transport mechanisms. In addition, a liquid polyphenyl ether membrane reinforced with a microporous support was shown to give similar results. The predominant ions produced in an ion trap mass spectrometer using electron ionization (EI) were mit 57 ([M - Cl](+)) and 62/64 ([M - CH2O](+)) while those produced under chemical ionization (CI) conditions were mit 57 ([M+ H - HCl](+)), 63/65 ([M + H - CH2O](+)), and 93/95 ([M + H](+)). Further, the hydrochloric acid-catalyzed ring opening of epichlorohydrin to yield 3-chloro-1,2-propanediol and other products was monitored on-line using the polyphenyl ether liquid membrane. The ability to perform continuous monitoring provides access to real time kinetic data unavailable through off-line chromatographic techniques.