The paper presents data on nickel(II) extraction from sulphate solutions with nonane-diluted bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HR). An IR spectroscopy method is used to examine the state of HR in nonane, and it was shown to exist as a monomer up to a concentration of 0.1 M. With a further increase in concentration, the acid forms associated species. The value of the concentration of the monomeric form is determined as a function of the total HR concentration in nonane in the range of 0.0-1.0 M. Based on the comparative analysis of the extraction equilibrium data together with information from the visible and IR absorption spectra of the nickel extract, the configuration of the nickel(II) complex in the organic phase was determined to be a low-spin, square-planar complex. The complex composition was found to correspond to the formula NiR 2 . The complex is hydrated, with the water very likely being bonded in the outer coordination sphere.