In the presence of catalysts, polycarboxylic acids such as meso-1,2,3,4-butanetetracarboxylic acid (BTCA) can cross-link cellulose in a process believed to involve cyclic anhydrides as intermediates. This study was undertaken to explore, in the absence of cellulose, interactions between catalysts and polycarboxylic acids that might explain the catalysts’ role. Aqueous solutions containing equimolar amounts of BTCA and a catalyst (NaH2PO4 or NaH2PO2) were evaporated to constant weight. The residues were subjected to thermal analysis and to infrared spectroscopic analysis after heating. As the BTCA-NaH2PO4 residue was heated, the BTCA remained complexed with phosphate until a monoanhydride formed-the same monoanhydride that formed when BTCA alone was heated to a higher temperature. In the BTCA-NaH2PO2 system an anhydride apparently formed at an even lower temperature than in the phosphate-containing system but then reacted with the hypophosphite.