Substituted isoindolones I ##STR1## where R.sup.1 and R.sup.2 =H, unsubstituted or substituted alkyl, alkenyl, alkynyl, cyano, C.sub.1 -C.sub.6 -alkoxycarbonyl, formyl, C.sub.1 -C.sub.6 -alkylcarbonyl, unsubstituted or substituted phenylcarbonyl, C.sub.3 -C.sub.8 -cycloalkyl, or, if R.sup.1 =H, R.sup.2 additionally=unsubstituted or substituted phenyl, orR.sup.1 and R.sup.2, together with the carbon atom to which they are bonded, are an unsubstituted or substituted 3- to 8-membered carbo- or heterocycle;where R.sup.3 to R.sup.6 have the meanings given in the specification. The compounds are useful as herbicides and for the desiccation/defoliation of plants.