AbstractCopolymerization of aldehydes has been carried out in the presence of anionic and cationic catalysts. Triisobutylaluminum gave an elastic copolymer of formaldehyde and acetaldehyde which did not melt until above 300°C. and did not dissolve in any organic solvents. The rate of the copolymerization of formaldehyde and acetaldehyde at −78°C. was very rapid, the reaction going to completion within 30 min. The thermal stability of the copolymer of formaldehyde and acetaldehyde was better than that of polyacetaldehyde but was inferior to that of polyformaldehyde; it reaches a minimum in the composition range of 40/60 to 60/40 of formaldehyde and acetaldeh