Display omitted •g-C3N4 (CN) was synthetized from melamine in air and nitrogen.•CN prepared in air contained the lower content of oxygen due to decarboxylation.•Oxygen was present mainly in the form of hydroxyl groups.•Influence of oxygen on the CN properties was investigated. Two types of bulk graphitic carbon nitrides (CN) were synthetized by heating melamine at 550 °C for 4 h in the air (CN/air) and nitrogen (CN/N2) atmosphere for us to find the effect of present oxygen on their properties. Bulk CN was exfoliated by further heating at 500 °C for 1–3 h in air and nitrogen as well. The obtained materials were studied in terms of structural, textural and physico-chemical properties including photocatalytic activity. The CN/air materials contained less oxygen (2.3–5.0 wt%) than the CN/N2 ones (about 7 wt%) due to decarboxylation during their synthesis and exfoliation in air. The CN/N2 materials had more structural defects, which were attacked by oxygen from the moment the materials came into contact with air. In both types of materials oxygen was supposed to be present mostly in the form of hydroxyl groups. As a result, the CN/N2 materials were slightly exfoliated and their surfaces were more hydrophilic than the CN/air ones. The photocatalytic activity of CN materials was dependent on their specific surface area (SSA). It was found that native oxygen existed in the CN materials and was important for their properties.