Herbicidal compounds of formula (I) and salts thereof, where A is --N= or --CH=; X is --OH, --OR10, --SH, --SR10, --SCN, NH2, --NHacyl, --CH2 OH, --CH2 OR9 or --CN; R1 and R2, which may be the same or different, each represent alkyl, alkoxy, haloalkyl, haloalkoxy, halo, alkylamino or dialkylamino; R3 is --CN, --COOR5, --CONR6 R7, --CSNH2, --CHO, --CH=Z, --CH(Oalkyl)2, --CH2 OH, --CH2 OR9, --COSR4a, --CS2 R4a, or a substituted or unsubstituted 5- or 6-membered heterocyclic group linked via a ring carbon atom which is between two ring heteroatoms; R4 and R4a, which may be the same or different, are each H, or a substituted or unsubstituted alkyl, alkenyl, alkynyl, cycloalkyl, aryl, aralkyl or heteroaryl group; R5 is H, or a substituted or unsubstituted alkyl, alkenyl, alkynyl, cycoalkyl, aryl, aralkyl or heteroaryl group, or a group of formula (II) where B and D are each alkyl, or together with the carbon atom to which they are attached form a 5-, 6- or 7-membered carbocyclic or heterocyclic ring; R6 is a group as defined for R4a, and R7 is a group as defined for R4a or is --SO2 R8, --OH, --CN, --OR10, --NH2 or --NHR10 ; or R6 and R7 together form a ring; R8 is a substituted or unsubstituted alkyl alkenyl, alkynyl, cycloalkyl, aralkyl, aryl or heteroaryl group, or a group --NR4b R4c where R4b and R4c, which may be the same or different, are each a group as defined for R4a, or together form a ring; R9 is a substituted or unsubstituted alkyl, alkenyl, alkynyl, cycloalkyl, aralkyl or acyl group; R10 is a substituted or unsubstituted alkyl, alkenyl, alkynyl, cycloalkyl, aryl, aralkyl, heteroaryl or acyl group; Z is =NOR4a or =N--NR4d R12, where R4d is a group as defined for R4a, and R1 is a group as defined for R4a or is a substituted or unsubstituted acyl group, or R4d and R12 together form a ring; with the proviso that, when R4 is ortho-substituted phenyl or naphthyl, any ortho-substituent thereon is halogen, --NO2, --OH, --OR10, --SH, --SR8, --SOR8, --SO2 R8, --NH2, --NR6 R10, aryl or heteroaryl.