Molecular confinement on solid surfaces can have important consequences for chemical reactivity. Pyrolysis of silica-immobilized 1,3-diphenylpropane (DPP) at 375 degreesC has been examined in the presence of a series of isomeric (by point of attachment) co-attached hydroaromatic spacer molecules to explore the role of molecular orientation on a free-radical reaction under surface confinement. The DPP pyrolysis rate was found to be sensitive to both the structure of the spacer and its attachment orientation to the surface. Spacer molecules with a meta orientation of the benzylic hydrogens with respect to the surface linkage were found to produce faster reaction rates than the corresponding para-oriented spacers. Molecular modeling indicates that the meta-attached spacers are better able to attain a favorable separation and geometry for hydrogen transfer to intermediate surface-con fined benzyl radicals in the hydrogen transfer, radical relay mechanism on the surface. These orientation effects have important implications in many fields including catalysis, solid-phase synthesis, and the design and preparation of new nanostructured materials.