Process for the preparation of herbicidally active substituted cyclohexan-1,3,5-triones which comprises either(a) reacting a compound of formula (II) ##STR1## wherein R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are, for example, hydrogen or C.sub.1 -C.sub.4 alkyl with a compound of formula (III):R.sup.1 COCN (III) wherein R.sup.1 is optionally substituted aryl in the presence of a base and a Lewis acid; or(b) reacting a compound of formula (VI) ##STR2## with a compound of formula (X) ##STR3## in the presence of a Lewis acid; or (c) where R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are the same, reacting a compound of formula (XI) ##STR4## with a compound of formula R.sup.2 X' in the presence of a base, wherein X' is a leaving group.