Carboxamides of the formulae Ia to Ic ##STR1## where X is oxygen or sulfur;R.sup.1 is hydrogen or substituted or unsubstituted cycloalkyl or alkyl;R.sup.2 is hydroxy, alkoxy, cyanoalkyl, substituted or unsubstituted alkenyl, alkynyl, phenyl or naphthyl, a substituted or unsubstituted 5- to 6-membered heterocyclic structure or one of the groups stated for R.sup.1, orR.sup.1 and R.sup.2 together are a 4- to 7-membered chain which contains, in addition to methylene groups, one of the following groups as ring member: oxygen, sulfur, N-methyl or carbonyl;R.sup.3 and R.sup.4 are each nitro, cyano, halogen, substituted or unsubstituted amino, alkoxy, alkylthio, a substituted or unsubstituted 5- to 6 -membered heterocyclic structure, substituted or unsubstituted alkenyl, alkynyl, phenyl, phenoxy or phenylthio, or one of the groups stated for R.sup.1 ;R.sup.5 is formyl, 4,5-dihydrooxazol-2-yl or COYR.sup.6 (Y=O,S);R.sup.6 is hydrogen, cycloalkyl, substituted or unsubstituted alkyl, alkenyl, alkynyl, cycloalkenyl or phenyl,a 5- to 6-membered heterocyclic structure, phthalimido, tetrahydrophthalimido, succinimido, maleimido, benzotriazolyl ora group --N.dbd.CR.sup.7 R.sup.8, whereR.sup.7 and R.sup.8 are hydrogen or alkyl, and R.sup.8 may also be cycloalkyl, phenyl, furyl or R.sup.7 and R.sup.8 together form a 4- to 7 -membered alkylene chain; and if R.sup.5 is carboxyl, methoxycarbonyl or ethoxycarbonyl and R.sup.2 is hydrogen, R.sup.3 is not hydrogen and R.sup.4 is not hydrogen or methyl, and if R.sup.5 is carboxyl, methoxycarbonyl or ethoxycarbonyl and R.sup.4 is hydrogen, R.sup.3 is not hydrogen or R.sup.2 is not one of the following groups: hydrogen; C.sub.1 -C.sub.4 -alkyl, phenyl; 2-(3,4-dimethoxyphenyl)ethyl or 2,5 -dichlorothien-3-yl;processes for the manufacture of compounds I and herbicidal agents containing them.