The impact of restricted mass transport on high-temperature, free-radical reactions has been explored through the use of organic compounds immobilized on silica surfaces by a thermally robust Si-O-C-aromatic linkage. The rate of thermolysis of surface-immobilized 1,3-diphenylpropane (approximate to DPP) at 375 degrees C under vacuum, by a free-radical chain pathway, was found to be very sensitive (factor of 40 variation) to the structure and orientation of a second, neighboring spacer molecule on the surface. Compared with the inert aromatic spacers, (e.g. biphenyl) it was found that spacer molecules containing reactive benzylic C-H bonds (e.g. diphenylmethane) are capable of accelerating the approximate to DPP thermolysis by a process that is unique to diffusionally constrained systems. A mechanism involving rapid serial hydrogen transfer steps on the surface is proposed, which results in radical intermediates being relayed across the surface and, hence, overcoming classical diffusional limitations.