The photocycloadditions of 5-X-adamantan-2-ones (5-X-AD, where X = F, Cl, Br, OH, Ph, and t-Bu) with fumaronitrile have been studied in acetonitrile and in aqueous solutions. When X is Cl, Br, Ph, or t-Bu, irradiation of an aqueous solution containing β-cyclodextrin (β-CD) leads to a dramatic reversal in face selectivity compared to that found in acetonitrile and water; however, there is no significant change in product ratio compared to that found in aqueous solution in the presence of α- and γ-CD. The effect observed with β-CD is interpreted with the assumption that the carbonyl π face syn to the bulky 5-substituent is partially blocked by the torus of the host due to complexation of the AD and CD. 1H NMR and concentration dependence studies provide support for this interpretation. Similar reversals of face selectivity upon complexation with (β-CD are observed in the thermal reduction of these ketones by sodium borohydride. It is also noted that a much enhanced syn hydride delivery occurs in water as compared to organic solvents and that this enhancement vanishes upon cooling.