Isoindoline pigments of the formula ##STR1## or of a formula which is a tautomer thereof, wherein R represents a direct bond or an aliphatic, cycloaliphatic, aliphatic-cycloaliphatic, araliphatic or aromatic radical, R.sub.1 represents a hydrogen atom or an alkyl group of 1 to 4 carbon atoms, or, if R represents an ethylene group, both R.sub.1 s together also represent an ethylene group, R.sub.2 and R.sub.3 represent hydrogen atoms, alkyl groups of 1 to 4 carbon atoms or phenyl groups which are unsubstituted or substituted by methyl groups or halogen atoms, X represents an oxygen atom or a NH group, and Y represents an oxygen or a sulfur atom or a NH group dye plastics in fast yellow or brown shades.