Studies of the thermal and photocatalytic properties of single crystal materials have proven to be useful in elucidating how surface structure influences photocatalytic activity for a range of reactions. While the use of well-defined single-crystal surfaces as model photocatalysts has many advantages, this approach also has some limitations since they do not allow one to study cooperative effects between different exposed crystal planes and how preferential migration of photogenerated charge carriers to different surfaces influences activity. To overcome these limitations while still using materials with well-defined surfaces, the research described in this dissertation seeks to understand the relationships between nanostructure and the thermal and photocatalytic properties of well-defined metal oxide nanocrystals. This approach is aided by recent advances in the synthesis of oxide nanocrystals which provide a high degree of control over crystallite size and shape. Furthermore, recent studies have reported that the dynamics of photogenerated charge carries and the consequent photocatalytic properties of a material may be significantly altered by decorating their surface with Pt. In this dissertation research project, thin films of various size- and shape-selected TiO2 nanocrystals, both with and without Pt decoration, are studied by using traditional surface science techniques. The thermal- and photocatalytic reactions of acetaldehyde, methanol, and acetic acid on these nanocrystal films were investigated using temperature programmed desorption in ultra-high vacuum. Results show that nanocrystal size and shape both have clear effects on the thermal and photocatalytic pathways occurring on the surfaces of the nanocrystals. Results also give insight into how Pt affects the dynamics of photogenerated charge carriers within a material. This dissertation demonstrates the efficacy of using well-defined nanostructured materials to gain understanding about how various parameters, such as a particle’s shape, size and precious metal surface decoration, can influence the catalytic properties of a material.