Conventional Y-CNx modified with appropriate co-catalysts is known for its ability to produce H2 in presence of a sacrificial reducing agent under visible light irradiation. Pristine Y-CNx is inactive for photocatalytic HER under monochromatic LED light irradiation at 420 nm (Fig. ). However, after deposition of the {Mo3} clusters and especially Pt nanoparticles, it demonstrates a very good performance reaching a H2 production of around 0.04 mmol in 7 h (Fig. ). Although the experimental data exposed and discussed above indicate that the basic structure of conventional yellow PCN is preserved also in the R-CNx material, its photocatalytic behaviour is very different. The activity of the red R-CNx modified with {Mo3} clusters or Pt nanoparticles is lower by an order of magnitude compared to Y-CNx, reaching only 0.005 mmol H2 after 7 h of irradiation (Fig. ). Similar to the conventional PCN counterpart, bare R-CNx without a co-catalyst does not photocatalyze HER under 420 nm light irradiation (Fig. ). Contrary to the case of Y-CNx, R-CNx shows higher photocatalytic activity if modified with the {Mo3} clusters than with Pt nanoparticles (Fig. ). A similar observation was reported by Wang et al. for protonated mesoporous carbon nitride materials. These results suggest that the surface properties of the R-CNx sample are different to a significant extent from those of Y-CNx, affecting the interaction of the former with co-catalysts. The photocatalytic activity of R-CNx under 420 nm irradiation is inferior to that of its yellow counterpart, nonetheless the appearance of an additional absorption band in the visiblelight range in the UV-vis spectrum of R-CNx (Fig. ) prompted us to study its performance also under excitation with energy light at the wavelength of 470 nm . Under these conditions, the Y-CNx sample is, as expected, completely inactive in photocatalytic HER both with and without co-catalysts (Fig. ), since it does not absorb in this spectral region (Fig. ). Intriguingly, the red R-CNx is not only capable of sacrificial H2 evolution under irradiation at 470 nm when modified with {Mo3} or Pt nanoparticles, but the pristine R-CNx without any co-catalysts even shows a superior photocatalytic activity (Fig. ). Therefore, the new functional groups in the red carbon nitride, the azo-linked poly(heptazine/triazines) formed as the result of the hightemperature treatment, are not only responsible for the strong red shift of the optical absorption edge of the material, but they also might possibly play an additional role of an active site for H2 evolution. Despite the rather moderate activity of this material, its ability to absorb a significant portion of the visible light spectrum and the metal-free nature of its active site for photocatalytic HER represent intriguing aspects of R-CNx with respect to solar light-driven photocatalysis. From the R-CNx catalyzed H2 evolution plots one can observe an apparent deactivation trend (Fig. ). We hypothesized that this was due to the weak reductive power of methanol as a sacrificial reducing agent, hence some stronger reductants were added to the R-CNx suspensions. The addition of ascorbic acid improved the photocatalytic performance of R-CNx modified with {Mo3} and Pt nanoparticles up to the level of the pristine catalyst, while leaving the activity of the co-catalyst modified samples unchanged (Fig. ). The introduction of another strong reductant, triethanolamine (TEOA), affected the activity of bare R-CNx only slightly, but had a strong impact on the activity of co-catalyst-modified R-CNx (Fig. ). Most notably, the strong deactivation trend could be reversed for the {Mo3}-modified R-CNx photocatalyst in the presence of TEOA, leading to a stable rate of H2 evolution (green trace in Fig. ). This might be attributed to a more effective oxidation of TEOA compared to ascorbic acid since positively charged TEOA at pH 10.5 (pKa = 7.9) is expected to adsorb more effectively at the negatively charged R-CNx than non-dissociated ascorbic acid (pKa = 4.3) at pH 2.4.