Compounds of the formula I (I), in which Ar is a group or unsubstituted or substituted cyclopentyl, cyclohexyl, naphthyl, biphenylyl or an O-, S- or N-containing 5- or 6-membered heterocyclic ring; R1 and R2 are C1-C20alkyl, OR11, CF3 or halogen; R3, R4 and R5 are hydrogen, C1-C20alkyl, OR11 or halogen; R6 is unsubstituted or substituted C1-C24alkyl, C2-C24alkyl, which is interrupted by O, S or NR14 and is unsubstituted or substituted; C2-C24alkenyl, uninterrupted or interrupted by O, S or NR14 and unsubstituted or substituted; unsubstituted or substituted C7-C24arylalkyl; C4-C24cycloalkyl, uninterrupted or interrupted by O, S and/or NR14; or C8-C24arylcycloalkyl; R11 is C1-C20alkyl, C3-C8cycloalkyl, phenyl, benzyl or C2-C20alkyl, interrupted by O or S and unsubstituted or substituted; R12 and R13 are hydrogen, C1-C20alkyl, C3-C8cycloalkyl, phenyl, benzyl or C2-C20alkyl, interrupted by O atoms and unsubstituted or substituted; or R12 and R13 together are C3-C5alkylene, uninterrupted or interrupted by O, S, or NR14; R14 is hydrogen, phenyl, C1-C12alkyl or C2-C12alkyl, interrupted by O or S and unsubstituted or substituted; and M is hydrogen, Li, Na or K; are valuable intermediates for the preparation of unsymmetrical bisacylphosphine oxides and monoacylphosphine oxides.