nd a fairly small amount of it can be directly liquefied from the reaction gas by partial condensation. Organic solvents absorb maleic anhydride contained in the reaction gas. More than 98% of the anhydride can be absorbed in this way. The solvent–anhydride mixture is then subjected to fractional distillation to separate maleic anhydride from the solvent, and the latter is returned to the absorption column (Lohbeck et al. 1990). The thus obtained pure maleic anhydride is then hydrolyzed into maleic acid according to the reaction equation