A combined computational and experimental study has been performed on the thermal [(pi)2(s) + (sigma)2(s) + (sigma)2(s)] cycloaddition reactions of quadricyclane with acetylene, ethylene, dicyanoacetylene, and dimethyl acetylenedicarboxylate (DMAD). The energy surfaces for the reactions were obtained using hybrid density functional theory at the B3LYP/6-31G(d) level. All attempts to locate concerted pathways for the reactions failed, since the putative transition structures for these processes correspond to second-order saddle points. Nonconcerted pathways, proceeding via biradical intermediates, were found for these reactions using unrestricted B3LYP/6-31G(d) theory. Intramolecular secondary kinetic isotope effects (SKIEs) were determined experimentally for the reaction of quadricyclane-1-d(1) and quadricyclane-1,5-d(2) with DMAD in benzene at 95 degrees C. The results were compared with the intermolecular SKIE for the reaction of quadricyclane-1,5,6,7-d(4) with DMAD under the same conditions. The k(H)/k(D1) and k(H)/k(D2) values are in quits good agreement with calculated values for the nonconcerted biradical mechanism, The experimental intermolecular SKIE for the addition of DMAD to quadricyclane-d(0) or -d(4) is negligible. This observation conflicts with either concerted or biradical mechanisms. Alternatives are discussed.