TiO2/g-C3N4 photocatalysts with the ratio of TiO2 to g-C3N4 ranging from 0.3/1 to 2/1 were prepared by simple mechanical mixing of pure g-C3N4 and commercial TiO2 Evonik P25. All the nanocomposites were characterized by X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, Raman spectroscopy, infrared spectroscopy, transmission electron microscopy, photoelectrochemical measurements, and nitrogen physisorption. The prepared mixtures along with pure TiO2 and g-C3N4 were tested for the photocatalytic reduction of carbon dioxide and photocatalytic decomposition of nitrous oxide. The pure g-C3N4 exhibited the lowest photocatalytic activity in both cases, pointing to a very high recombination rate of charge carriers. On the other hand, the most active photocatalyst toward all the products was (0.3/1)TiO2/g-C3N4. The highest activity is achieved by combination of a number of factors: (i) specific surface area, (ii) adsorption edge energy, (iii) crystallite size, and (iv) efficient separation of the charge carriers, where the efficient charge separation is the most decisive parameter.