<p>1,3-Dialkylimidazolium carboxylate ionic liquids (ILs) have recently undergone a surge of interest as media for spinning of cellulosic fibers. For 1-alkyl-3-methylimidazolium acetates, the acetylation of cellulose in the process of dope preparation and spinning is a recognized side reaction. In this work, acylation of cellulose by the carboxylate anions of 1-ethyl-3-methylimidazolium-based ILs was shown to be a general side reaction, studied in detail for a series of the corresponding acetates, propanoates and octanoates. The acylation does not proceed in neat, freshly purified ILs, but requires free imidazole which is present in aged (long-term stored) or thermally stressed ILs as a common degradation product of 1,3-dialkylimidazolium cations. Imidazole (Im), in combination with the free acids from the conjugated carboxylate anions, forms <i>N</i>-acylimidazoles as well-known potent, catalytically acting acylation reagents which also effect acylation of cellulose and related carbohydrates. <i>N</i>-Methylimidazole (MIm), another common aging product of 1-alkyl-3-methylimidazolium cations, cannot form similar acyl transfer reagents and thus does not have such a catalytic effect. The amount of resulting esters in cellulose samples spun from aged (thermally stressed and/or long-term stored) 1,3-dialkylimidazolium carboxylates (acetate, pentanoate, octanoate) was determined, according to an established method for accurate quantification of small amounts of acetate esters in biomaterials, based on Zemplén saponification/transesterification. The reactivity decreases with the chain length of the carboxylate anion: a ternary IL mixture that contains the anions acetate, pentanoate and octanoate in a molar ratio of 1:1:1 caused acylation in an approximate molar ratio of 16:4:1. The underlying reaction mechanisms and ways to minimize unwanted cellulose acylation as side reactions are discussed.</p>