(+/-)-bis-Diethyl 4-(trimethylstannylmethyl)-bicyclo[3.3.0]octane-2,2-dicarboxylate, C18H32O4Sn, M(r) = 431.14, triclinic, P1BAR a = 7.674 (2), b = 11.113 (3), c = 13.346 (4) angstrom, alpha = 71.64 (2), beta = 79.49 (2), gamma = 86.62 (2)-degrees, V = 1062.1 (5) angstrom3, Z = 2, D(x) = 1.348 Mg m-3, lambda(Mo K-alphaBAR)= 0.71069 angstrom mu = 1.22 mm-1, F(000) = 444, T = 293 K, R = 0.055 and wR = 0.063 for 2526 observed reflections. Trimethyltin hydride adds to the terminal double bond of 1,6-dienes and the resulting radical leads to exo-trigonal ring closure. The major product was analyzed by X-ray crystallography to determine the relative configuration of the ring junction which could not be established unambiguously by H-1 NMR studies. The cis-fused bicyclo[3.3.0] system contains a trimethylstannylmethyl side arm which is in the concave face of the molecule.