The immobilization reaction of benzenesulfonyl chloride (BSC) from the gas phase onto aminosilane-modified silica surfaces has been characterized by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) measurements. Porous (60-Angstrom diameter pores) and nonporous silica substrates were investigated. The silane modification was accomplished by a gas-phase adsorption of 3-aminopropyltrimethoxysilane (APS). The nitrogen Is (N1s) XPS photopeaks of the modified substrates exhibit components at 398.8 and 401.0 eV, assignable respectively to free and protonated primary amine groups. Upon subsequent adsorption of BSC, a new signal from sulfur is observed at a binding energy of 168.8 eV, similar to values observed previously for sulfonamide compounds. Additionally, the 398.8-eV N1s photopeak decreases in intensity, and an additional component appears at 399.9 eV, consistent with the transformation of the primary amine to an amide functionality. FTIR spectra of APS-modified surfaces exhibit bands at 3298 and 3367 cm(-1), due to the symmetric and asymmetric N-H stretching modes, and at 3176 cm(-1) due to the -NH2 scissor overtone. Upon immobilization of BSC, the two bands associated with N-H stretch modes of the primary amine collapse to a single broad band at 3282 cm(-1), consistent with the formation of an amide. Also, a number of bands associated with phenyl vibrations are observed, evincing the presence of BSC on the surface. From the FTIR and XPS data it is observed that varying extents of BSC coverage are obtained from similar experiments. Generally, BSC coverage is less on the porous silica, suggesting that the pore structure may hinder diffusion of BSC into the pores. Residual protonated amine groups are consistently observed following the reactions, which may also prevent complete BSC coverage.