Abstract Two compounds of stoichiometry Cu(CH3COO)2·L (L=triphenylphosphine or triphenylphosphine oxide) were prepared. These compounds display the antiferromagnetic behavior typical of dimeric copper(II) carboxylates. The singlet-triplet splitting energy is 371 cm−1 for the phosphine adduct and 319 cm−1 for the phosphine oxide adduct.