The title diphosphine 1 has been synthesized by double Michael addition of diphenylphosphine to 4,4-dimethyl-2,5-cyclohexadienone (2). NMR spectroscopy revealed that instead of the desired cis adduct 1a, which is of interest as a bidentate ligand for transition metal centers, the trans isomer 1b had been obtained. This was rationalized by analyzing the stereochemistry of intermediates in this reaction. Attempted conversion of 1b to the thermodynamically more stable 1a by oxidation to the bisphosphine oxide 5 followed by base catalyzed equilibration was thwarted by decomposition in the second step. © 2001 Elsevier Science Ltd. All rights reserved.