Laboratory-prepared poly(divinylbenzene-co-ethylvinylbenzene resin and the commercial version, Amberlite XAD-4, were modified by the anti-Markovnikov addition of thiols to the pendant vinylbenzene groups, linking functional groups through a thioether-dimethylene spacer to the resin. Compared to lab-made resins, Amberlite XAD-4, which has a much higher surface area and porosity, could be thiol-modified to a greater extent. Disulfide addition was then done to the pendant vinylbenzene groups of XAD-4 exclusively. Depending on the thiol or disulfide used, different solvents and initiators were employed to optimize functionalization. The most important factors for maximum modification were: the solubility of the thiol or disulfide, the accessibility of vinyl groups and, in the case of the disulfide reactions, the ability to form the sulfenyl iodide intermediate. The degree of functionalization of thiol-modified XAD-4 was as much as 25%, up to 76% of vinyl groups were converted to thioether groups; whereas with disulfide-modified XAD-4, the degree of functionalization was as much as 18%, representing a conversion of vinylbenzene groups to (1,2-dithioether-ethyl)benzene functional groups by as much as 55%.