IR, NMR (H,13C and31P) and electronic spectroscopies have been used to study the tautomeric forms of 3-phenyl-4-benzoyl-isoxazol-5-one, "HPBI", and its interaction with tri-rc-octylphosphine oxide, ‘TOPO’, in wet toluene or benzene, and in chloroform. These metal extracting media have been compared with analogous solutions containing l-phenyl-3-methyI-4-acyI-pyrazoI-5-oIs. The spectroscopic results are compared with those of molecular modelling studies. Spectroscopic criteria that allow a choice between the various tautomeric forms of pyrazolones or isoxazolones are briefly reviewed.