The development of a solid-state nuclear magnetic resonance (NMR) method allowing the quantification of active sites (i.e., residual vinyl groups) accessible for chemical functionalization on the surface of poly(divinyl benzene) microspheres is presented. Residual vinyl groups of poly(divinyl benzene) microspheres (PDVB55 and PDVB80) were quantified via solid-state C-13 cross-polarization magic-angle spinning (CP-MAS) NMR spectroscopy. In addition, C-13 CP-MAS NMR spectroscopy allows the comparison of core and grafted microspheres functionalization on the same (arbitrary) scale in a short measuring time. This scale was calibrated by an extended absolute quantification of the vinyl groups using C-13 single pulse excitation (SPE) MAS NMR spectroscopy. The degree of cross-linking of the microspheres was calculated to be 30 and 50% for PDVB55 and PDVB80 microspheres, respectively. The number of active groups per nominal surface area is 110 and 179 groups per mm(2) for PDVB55 and PDVB80 microspheres, respectively. The loading capacities of the microspheres (e.g., 0.61 and 0.65 mmol . g(-1)) are not too far removed from those found in Merrifield resins of comparable sizes.