An electron spin resonance study of ClO2and Cl2-adsorbed on faujasite and mordenite has shown that the surface environment can cause significant changes in the spin hamiltonian parameters describing these systems. In the case of faujasite, two distinct adsorption sites are observed. It is shown that the experimental hyperfine shifts can be explained by the presence of strong electric fields, presumably due to the cations in the zeolite structure.