In the first of a two-part series on Friedel-Crafts modified polystyrene (FCMPS), synthesis and characterization of FCMPS are described, including aqueous- and vapor-phase sorption equilibrium isotherms. The effect of synthesis variables on the final polymer properties was studied, including the choice of the cross-linking agent, percent divinylbenzene (DVB) in the poly(styrene/divinylbenzene) (PSDVB) starting polymer, percent total cross-linking, and polymer concentration. The most effective volatile organic compound (VOC) sorbent is formed when the FCMPS is synthesized at 100% "theoretical" cross-linking with chloromethyl methyl ether or chloromethyl ethyl ether using a lightly cross-linked starting PSDVB (<2.5% DVB). Both the anomalous swelling behavior and significant absorption contributions to VOC sorption were verified for FCMPS. An alternative explanation for the swelling behavior is proposed. Part 2 of the series compares custom-synthesized FCMPS and commercial sorbents, including detailed isotherm analysis.