Compounds of the formula I ##STR1##

in which

Ar is a group ##STR2##

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.