Propanephosphonic acid anhydride (T3P) has been proposed as a novel reagent for the preparation of maleic acid isoimides from the corresponding monoamides. A series of substituted aromatic and aliphatic isoimides have been prepared in good yields. The main advantage of this synthetic method is the use of environmentally benign, cost-efficient reagents and solvents, which are also safer to handle than the ones employed previously. ... Supplemental materials are ...