Two carbon-bridged cyclodeoxyadenosines, 2'-deoxy-8, 2'-methanoadenosine (8, 2'-MA) and 2'-deoxy-8.2'-ethanoadenosine (8, 2'-EA), were investigated by X-ray and nuclear magnetic resonance (NMR) analyses. Both compounds crystallize in the orthorhombic system with space group P212121. The unit-cell dimensions are a=9.427 (1), b=17.287 (2)and c=6.908 (1)A for 8, 2'-MA and a=15.599(4), b=19.468 (5) and c=9.955 (3) A for 8, 2'-EA. Both structures were solved by direct methods and refined to R values of 0.037 for 8, 2'-MA and 0.052 for 8, 2'-EA. There are two independent cyclonucleoside molecules and one isopropanol molecule per asymmetric unit in the crystal of 8, 2'-EA. The glycosidec torsion angles, x, O(4')-C(1')-N(9)-C(4), are -68.7 (5)° and -77.2(4)° for molecules (a) and (b) of 8, 2'-EA, respectively, and -65.7(4)° for 8, 2'-MA, and are in high-anti conformation (x; -90°--50°). The deoxyribose ring of 8, 2'-MA takes the C(3')-exo, C(4')-endo twist pucker, and the conformation about the C(4')-C(5') bond is gauche+.In contrast, both deoxyribose rings of 8, 2'-EA exhibit C(1')-exo envelope puckering, and the conformation about the C(4')-C(5') bond is gauche+ for molecule (a) and trans for molecule (b). 1H-NMR data indicate that both of the cyclonucleosides in solution exist in the equilibrium of several sugar and exocyclic conformations. 8, 2'-EA adopts two different conformations, even around the glycosidic bound.