We present the development of a method for the determination of methylene bridge crosslinking in C-13-labeled chloromethylated polystyrene-divinylbenzene resins. Our system uses a new room temperature reduction of the chloromethyl groups that circumvents the possible challenges associated with additional crosslinking. We demonstrate how the reduction of the chloromethyl groups allowed for the determination of methylene bridging, derived from the methylenebis(ethenylbenzene) crosslinker, based on the integration of the methylene signal in the C-13 NMR spectra. Utilizing this method, the total crosslinking within the chloromethylated resin generated at 35 degrees C was determined to be upward of 10 wt %, which increased from 6 wt % in the unfunctionalized resin. (C) 2016 Wiley Periodicals, Inc.