Interest in photocatalysis driven by solar energy has risen greatly over the past four decades due to the fact that it is one of the most promising approaches to help solve the energy crisis and effectively combat environmental contamination. TiO2-based nanomaterials remain attractive candidates as photocatalysts due to their high photo-efficiency, strong oxidizing power, good stability, and non-toxicity. Great attention has been given to improving its conventional (UV-driven) performance by exploiting, inter alia, the synergy of mixed (anatase/rutile) phase junctions, or making more perfect crystals (defects act as charge recombination centers) without sacrificing texture (surface area) etc. In the last two decades, intensive studies have been devoted to overcoming arguably the greatest limitation of TiO2, viz., its insensitivity to visible light. Early attention was given to bulk doping with transition metal ions but many act as recombination centers. This led to a related strategy with anions, where some success has been achieved with nitrogen as dopant. However, no major breakthrough is forthcoming as yet and other approaches are attracting serious interest, e.g., sensitization by narrow-band gap semiconductors, decoration with nanoparticulate metals having plasmon resonance absorption, or organic polymers less susceptible to photo-bleaching than conventional dyes. Furthermore, while major efforts have been expended in materials synthesis, characterization and testing, spectroscopic investigations at the reactant/catalyst interfaces are sparse. In particular, in situ studies at the photocatalyst surface are vital to unravel the mechanistic detail, yielding key information integral to the design and development of new materials with superior performance. In the present work, two complementary techniques, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and Attenuated Total Reflection (ATR) Fourier Transform Infrared (FTIR) Spectroscopy have been used for in situ investigations of some typical photocatalytic reactions at the gas/solid and liquid/solid interfaces, respectively. Commercial material, Degussa P25 TiO2, was used mainly for system verification and fundamental studies on the state of material under band-gap excitation, especially ATR-FTIR, and as a benchmark (in pristine and metalized form) for novel visible-sensitized photocatalysts prepared in this thesis.