The g-C₃N₄/TiO₂ nanopowders prepared by the annealing of melamine and TiO₂ P25 at 550 °C were investigated under dark and upon UV or visible-light photoactivation using X- and Q-band electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of powders monitored at room temperature and 100 K showed the impact of the initial loading ratio of melamine/TiO₂ on the character of paramagnetic centers observed. For the photocatalysts synthesized using a lower titania content, the paramagnetic signals characteristic for the g-C₃N₄/TiO₂ nanocomposites were already found before exposure. The samples annealed using the higher TiO₂ loading revealed the photoinduced generation of paramagnetic nitrogen bulk centers (g-tensor components g₁ = 2.005, g₂ = 2.004, g₃ = 2.003 and hyperfine couplings from the nitrogen A₁ = 0.23 mT, A₂ = 0.44 mT, A₃ = 3.23 mT) typical for N-doped TiO₂. The ability of photocatalysts to generate reactive oxygen species (ROS) upon in situ UV or visible-light photoexcitation was tested in water or dimethyl sulfoxide by EPR spin trapping using 5,5-dimethyl 1-pyrroline N-oxide. The results obtained reflect the differences in photocatalyst nanostructures caused by the differing initial ratio of melamine/TiO₂; the photocatalyst prepared by the high-temperature treatment of melamine/TiO₂ wt. ratio of 1:3 revealed an adequate photoactivity in both spectral regions.