Phosphorus-containing benzoyl derivatives of the formula I where: P(═X)R1R2X is an organic phosphorus radical; R3 is hydrogen, nitro, cyano, halogen, unsub. or sub. alkyl, alkylcarbonyl, alkoxycarbonyl, unsub. or sub. alkoxy, unsub. or sub. alkylthio, unsub. or sub. alkylsulfinyl, unsub. or sub. alkylsulfonyl, unsub. or sub. aminosulfonyl, unsub. or sub. amino, P(═X)R1R2, unsub. or sub. phenyl or unsub. or sub. heterocyclyl; R4 is nitro, cyano, halogen, alkyl, alkoxy, alkylthio, alkylsulfinyl or alkylsulfonyl; l is 0, 1 or 2; Q is unsub. or sub. 1-hydroxy-3-oxocyclohex-1-en-2-yl, unsub. or sub. 5-hydroxypyrazol-4-yl, unsub. or sub. isoxazol-4-yl or unsub. or sub. 2-cyano-1-oxoeth-2-yl; and their agriculturally useful salts; processes and intermediates for preparing the phosphorus-containing benzoyl derivatives; compositions comprising them and the use of these derivatives or the compositions comprising them for controlling undesirable plants are described.