AbstractSeveral cosolvent mixtures for PMMA are reported. Their cosolvency has been confirmed by studying phase separation temperatures as a function of solvent composition. The results show that they are very powerful cosolvents. Acetonitrile forms such cosolvent mixtures with compounds containing different chemical groups (ester, ketone, alcohol, chloroalkane). The mixtures of acetonitrile with alcohols are nonsymmetrical with respect to the solubility parameter value of PMMA. They cannot be explained by simple δ theory. The mixture of formamide with ethanol is also nonsymmetrical and constitutes a true cosolvent pair for PMMA. The associating tendency of the polymer in the single solvent, plus the somewhat ordered structure of the pure liquids, are invoked to suggest that cosolvency is attained by breaking such order and association (in addition to the existence of unfavourable interactions between the cosolvents).