A process for the preparation of a mono(acyl)phosphine of the general formula I and/or a bis(acyl)phosphine of the general formula II, wherein R 1, R 2, R 3, R 4 and R 5 are the same or different and are independently selected from H, halogen, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl, C 1 -C 8 -alkoxy, C 2 -C 8 -alkenyloxy, C 3 -C 8 -cycloalkyl, C 6 -C 12 -aryl, C 3 -C 8 -cycloalkoxy, C 7 -C 12 -arylalkoxy, C 9 -C 15 -alkenylarylalkoxy, nitro-, C 6 -C 12 -arylsulfonyl, 4-alkylarylsulfonyl, C 1 -C 20 -alkylcarboxy, C 1 -C 8 -alkoxycarbonyl, SR 14, NHR 14 or NR 14 R 15 with R 14 and R 15 being independently selected from H, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl and C 3 -C 8 -cycloalkyl, and an O-, S-or N-containing 5- or 6-membered heterocyclic ring; R 6 is H or R 6 is replaced by an alkaline earth metal cation or a mixed alkali metal/alkaline earth metal cation; wherein R 1, R 2, R 3, R 4, R 5, R 7, R 8, R 9, R 10 and R 11 are the same or different and are independently selected from H, halogen, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl, C 1 -C 8 -alkoxy, C 2 -C 8 -alkenyloxy, C 3 -C 8 -cycloalkyl, C 6 -C 12 -aryl, C 3 -C 8 -cycloalkoxy, C 7 -C 12 -arylalkoxy, C 9 -C 15 -alkenylarylalkoxy, nitro-, C 6 -C 12 -arylsulfonyl, 4-alkylarylsulfonyl, C 1 -C 20 -alkylcarboxy, C 1 -C 8 -alkoxycarbonyl, SR 14, NHR 14 or NR 14 R 15 with R 14 and R 15 being independently selected from H, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl and C 3 -C 8 -cycloalkyl, and an O-, S- or N-containing 5- or 6-membered heterocyclic ring; the process comprising the steps of a) contacting a metal phosphide selected from the group consisting of Ca 3 P 2, Zn 3 P 2, AIP, Fe 3 P, Ni 3 P 2, Sr 3 P 2, Ba 3 P 2, Co 3 P 2, ScP, Ti 3 P 4, Sn 3 P 4, WP 2, LaP, Pb 3 P 2, BiP, and mixtures thereof or a mixed metal phosphide being an iron cobalt phosphide or a zirconium niobium phosphide, with a chelating agent, b) contacting the mixture obtained in step a) with a compound of the general formula IIIa and/or IIIb, wherein R 1, R 2, R 3, R 4 and R 5 and/or R 7, R 8, R 9, R 10 and R 11 are as defined above; Z is selected from halogen, C 1 -C 20 -alkylcarboxy, C 6 -C 12 -arylcarboxy, C 1 -C 8 -alkoxy and C 6 -C 12 -aryloxy; and c) and acidifying the mixture obtained in step b). wherein R 1, R 2, R 3, R 4 and R 5 are the same or different and are independently selected from H, halogen, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl, C 1 -C 8 -alkoxy, C 2 -C 8 -alkenyloxy, C 3 -C 8 -cycloalkyl, C 6 -C 12 -aryl, C 3 -C 8 -cycloalkoxy, C 7 -C 12 -arylalkoxy, C 9 -C 15 -alkenylarylalkoxy, nitro-, C 6 -C 12 -arylsulfonyl, 4-alkylarylsulfonyl, C 1 -C 20 -alkylcarboxy, C 1 -C 8 -alkoxycarbonyl, SR 14, NHR 14 or NR 14 R 15 with R 14 and R 15 being independently selected from H, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl and C 3 -C 8 -cycloalkyl, and an O-, S-or N-containing 5- or 6-membered heterocyclic ring; R 6 is H or R 6 is replaced by an alkaline earth metal cation or a mixed alkali metal/alkaline earth metal cation; wherein R 1, R 2, R 3, R 4, R 5, R 7, R 8, R 9, R 10 and R 11 are the same or different and are independently selected from H, halogen, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl, C 1 -C 8 -alkoxy, C 2 -C 8 -alkenyloxy, C 3 -C 8 -cycloalkyl, C 6 -C 12 -aryl, C 3 -C 8 -cycloalkoxy, C 7 -C 12 -arylalkoxy, C 9 -C 15 -alkenylarylalkoxy, nitro-, C 6 -C 12 -arylsulfonyl, 4-alkylarylsulfonyl, C 1 -C 20 -alkylcarboxy, C 1 -C 8 -alkoxycarbonyl, SR 14, NHR 14 or NR 14 R 15 with R 14 and R 15 being independently selected from H, linear or branched C 1 -C 20 -alkyl, linear or branched C 2 -C 8 -alkenyl and C 3 -C 8 -cycloalkyl, and an O-, S- or N-containing 5- or 6-membered heterocyclic ring; the process comprising the steps of a) contacting a metal phosphide selected from the group consisting of Ca 3 P 2, Zn 3 P 2, AIP, Fe 3 P, Ni 3 P 2, Sr 3 P 2, Ba 3 P 2, Co 3 P 2, ScP, Ti 3 P 4, Sn 3 P 4, WP 2, LaP, Pb 3 P 2, BiP, and mixtures thereof or a mixed metal phosphide being an iron cobalt phosphide or a zirconium niobium phosphide, with a chelating agent, b) contacting the mixture obtained in step a) with a compound of the general formula IIIa and/or IIIb, wherein R 1, R 2, R 3, R 4 and R 5 and/or R 7, R 8, R 9, R 10 and R 11 are as defined above; Z is selected from halogen, C 1 -C 20 -alkylcarboxy, C 6 -C 12 -arylcarboxy, C 1 -C 8 -alkoxy and C 6 -C 12 -aryloxy; and c) and acidifying the mixture obtained in step b). a) contacting a metal phosphide selected from the group consisting of Ca 3 P 2, Zn 3 P 2, AIP, Fe 3 P, Ni 3 P 2, Sr 3 P 2, Ba 3 P 2, Co 3 P 2, ScP, Ti 3 P 4, Sn 3 P 4, WP 2, LaP, Pb 3 P 2, BiP, and mixtures thereof or a mixed metal phosphide being an iron cobalt phosphide or a zirconium niobium phosphide, with a chelating agent, b) contacting the mixture obtained in step a) with a compound of the general formula IIIa and/or IIIb, wherein R 1, R 2, R 3, R 4 and R 5 and/or R 7, R 8, R 9, R 10 and R 11 are as defined above; Z is selected from halogen, C 1 -C 20 -alkylcarboxy, C 6 -C 12 -arylcarboxy, C 1 -C 8 -alkoxy and C 6 -C 12 -aryloxy; and c) and acidifying the mixture obtained in step b).