The photocatalytic activity of H2Ti4O9/CdS nanocomposites incorporating CdS particles, less than 0.8 nm thick, in the interlayer of H2Ti4O9 was evaluated for the reduction of NO3- with and without methanol. NO3- was photochemically reduced by bandgap illumination in the presence of H2Ti4O9/CdS nanocomposites with and without methanol although unsupported CdS showed no noticeable photocatalytic activity for NO3- reduction. The oxidation of CdS in H2Ti4O9/CdS to SO42- was accompanied by NO3- reduction without methanol, whereas addition of methanol was useful to promote the NO3- reduction and depress the oxidation of CdS. The catalytic activity of H2Ti4O9/CdS greatly increased with doping Pt particles in the interlayer.