Herbicidally active oxyguanidine derivatives of the formula ##STR1## in which M represents hydrogen or one equivalent of a metal,R.sup.1 represents halogen, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -halogenoalkyl, C.sub.1 -C.sub.4 -alkoxy, phenyl or C.sub.1 -C.sub.2 -alkoxy-carbonyl,R.sup.2 represents hydrogen or the radical ##STR2## and R.sup.3 represents C.sub.4 -C.sub.12 -alkyl, isopropyl or--if M represents one equivalent of a metal--also propyl, or represents C.sub.2 -C.sub.12 -halogenoalkyl, C.sub.1 -C.sub.4 -alkoxycarbonyl-C.sub.1 -C.sub.2 -alkyl, aminocarbonylmethyl, C.sub.1 -C.sub.4 -alkylaminocarbonylmethyl, di-C.sub.1 -C.sub.4 -alkylaminocarbonylmethyl, C.sub.3 -C.sub.12 -alkenyl, C.sub.3 -C.sub.12 -alkinyl, benzyl which is substituted by halogen, nitro, cyano, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy or C.sub.1 -C.sub.4 -alkoxycarbonyl, or phenyl, benzhydryl, or phenethyl, all optionally substituted by halogen, nitro, cyano, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy or C.sub.1 -C.sub.4 -alkoxycarbonyl, or carboxy-C.sub.1 -Ca.sub.2 -alkyl,or adducts thereof with strong acids.