Abstract: The cellulose solvent N-methylmorpholine-N-oxide monohydrate (NMMO) reacts violently with carbodiimide coupling agents, such as the widely used DCC (dicyclohexylcarbodiimide) and EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), which can cause explosions, heavy smoke generation and complete charring of the reaction mixtures. For this reason it is clear that cellulose or other carbohydrates dissolved in NMMO (“Lyocell conditions”) cannot be safely esterified or amidified under DCC/EDC conditions. It is important to note that cellulosic material regenerated from NMMO solutions also poses a risk if it contains residual NMMO, which can then come into contact with DCC-type coupling agents in non-aqueous media, as recent accidents have unfortunately shown. Similarly, EDC/DCC traces in cellulose can have detrimental effects when trying to dissolve the material in NMMO. Working in aqueous systems generally reduces the risk of uncontrolled reactions, while alcohols or aprotic solvents are not very effective in this regard. Washing for NMMO removal or EDC/DCC removal should therefore always be carried out by aqueous media, if any possible, if there is a potential risk that the reagents might come into contact later. This warning describes the potential hazards of the reagent combination, highlights the underlying degradation mechanism and explains the precautions to be taken to avoid any “exothermic events”. Graphic abstract: [Figure not available: see fulltext.].