The dimers on clean diamond (1 0 0)-2 x 1 are linked by a sigma bond and a highly strained T, bond. The weakness of the surface pi bond causes the reactivity of this surface to be intermediate between that of an alkene and a bi-radical, We illustrate this behavior by investigating two prototypical cycloaddition surface reactions using multiple-internal-reflection infrared spectroscopy. Adsorption of 1,3-butadiene occurs via [4 + 2] addition to surface dimers, analogous to Diels-Alder chemistry of alkenes. However, [2 + 2] cycloaddition of cyclopentene, a reaction that is symmetry-forbidden with alkenes, occurred with a sticking coefficient of similar to 10(-3). These reactions illustrate novel approaches to the functionalization of diamond surfaces.