Abstract: Periodateoxidation of cellulose to produce “dialdehyde cellulose” (DAC) has lately received increasing attention in sustainable materials development. Despite the longstanding research interest and numerous reported studies, there is still an enormous variation in the proposed preparation and work-up protocols. This apparently reduces comparability and causes reproducibility problems in DAC research. Two simple but prevalent work-up protocols, namely glycol quenching and filtration/washing, were critically examined and compared, resulting in this cautionary note. Various analytical techniques were applied to quantify residual iodine species and organic contaminations from quenching side reactions. The commonly practiced glycol addition cannot remove all oxidising iodine compounds. Both glycol and the formed formaldehyde are incorporated into DAC's polymeric structure. Quenching of excess periodate with glycol can thus clearly be discouraged. Instead, simple washing protocols are recommended which do not bear the risk of side reactions with organic contaminants. While simple washing was sufficient for mildly oxidised celluloses, higher oxidised samples were more likely to trap residual (per)iodate, as determined by thiosulfate titration. For work-up, simple washing with water is proposed while determining potential iodine contaminations after washing with a simple colorimetric test and, if needed, removal of residual periodate by washing with an aqueous sodium thiosulfate solution.