Abstract In an effort to better understand the nature of the species which may be involved in the selective extraction of actinides over the lanthanides – relevant to nuclear reprocessing – by 2,2′:6′,2″-terpyridine (terpy)-type ligands, in synergistic combination with a liphophilic carboxylic acid coextractant, the Pr III / PhCO 2 - / NO 3 - / terpy reaction system has been investigated. The investigation resulted in the isolation of the dinuclear complex [Pr 2(O 2CPh) 5(NO 3)(terpy) 2(H 2O)(MeOH)] ( 1), in which the metal ions are bridged by two η 1:η 1:μ 2 benzoate ligands. Omission of nitrates from the reaction mixture gives the structurally similar complex [Pr 2(O 2CPh) 6(terpy) 2(H 2O) 2] ( 2). IR and solid-state electronic spectra are discussed in terms of the nature of bonding and the known structures. No magnetic interaction was found between the Pr III ions in 2. The possible role of the carboxylic acid coextractant in the extraction process is discussed.