Here we report the synthesis of a series of new nanocrystalline TiO2-(MoO3) core-shell materials whose photoabsorption energy (PE, the energy required to excite TiOz-core valence band electrons to MoO3- shell conduction band states) properties are correlated with both the nanoparticle size and the degree of chemical interaction between the TiO2 core and the MoO3 shell. The TiO2-(MoO3) nanoparticle size can be readily adjusted from 80 to 40 Angstrom, and in this series, the PE decreases from 2.88 to 2.60 eV with decreasing particle sits. The systematic PE red-shift exhibited by the core-shell materials is ascribed to the change in the relative position of the MoO3-shell conduction band as it evolves from less than a monolayer to a two monolayer shell.