It has been demonstrated that porous sol-gel derived (SGD) alumina can function as an effective microorganized medium in which to perform size- or shape-selective organic transformations. For the photolysis of 7-dehydrocholesterol (7D) encapsulated within SGD-alumina, it was found that the adsorption characteristics of 7D and the pore size of the SGD-alumina greatly influenced the photochemical selectivity of the cis-trans isomerization of previtamin D-3 to tachysterol(3). An increase in the ratio of previtamin D-3 (P-3) to tachysterol(3) (T-3) was obtained on photolysis of 7D adsorbed within SGD-alumina with a pore size of 10-20 Angstrom. This result is attributed to a confinement effect that inhibits the cis-trans isomerization of P-3 to T-3. In addition to the confinement effect on the photochemical selectivity, it was found that the adsorption characteristics of 7D to the surface of SGD-alumina also play a critical role in the photochemical selectivity. The results indicate that confinement in addition to chemisorption via the 3-hydroxyl group of 7D to the surface of activated SGD-alumina is necessary to optimize the photochemical selectivity. The presented results also provide direct experimental evidence to demonstrate that adsorption solely to a metal oxide surface via the 3-hydroxyl group is not sufficient to inhibit the cis-trans isomerization of P-3 to D-3.