In this paper some aspects of the liquid-phase oxidation of ethylbenzene with air catalysed by Ni2+ complexes have been studied; the use of a nickel soluble complex without acetylacetone ligands gave unsatisfactory results. Quaternary ammonium salts (R4NBF4) (R = n-Bu, Me) were used as co-catalysts, and the ammonium salt with the longer radical n-Bu gave better results. On the other hand, a catalytic system has been designed based on Ni(acac)(2) and 1-n-butyl-3-methylimidazolium hexafluorophosphate and its reactivity has been explored in the oxidation with air of ethylbenzene at atmospheric pressure, showing that this catalytic system could be at the moment an alternative to the catalysts in use for this oxidation process. A new analytical procedure based on FT-IR spectroscopy to analyse the main reaction by-products, methylphenyl carbinol and acetophenone has also been described,