Thio-derivatives of P-diketones have received considerable attention in recent years and particular significance has been attached to the monothio-derivatives because of their ability to form chelates readily with certain divalent metals which show littlc or no reaction with the /I-diketoncs thcmselves'*2. The analytical utility of these chelates has not been fully exploited, although Berg and Reed' have examined thiothenoyltrifluoroacetone as a possible calorimetric reagent for zinc, cadmium, mercury(II), and lead. The volatility of the chelates of fluorinated monothio-/3diketones has been utilized in gas chromatography4 and separations of nickel, cobalt, and palladium5. and nickel. palladium, and platinum6 have been reported. Studies3*7*8 of the visible and ultraviolet spectra of some metal monothio-Pdikctonates have provided spectrophotometric data of considerable analytical significance, and recently some favourable solvent extraction properties have been described' which reinforce the general analytical potentialities of these ligands as useful analytical reagents. Earlier investigations4, particularly of the Group VIII metal chelates of these monothio-S-diketoncs, indicated that bis(monothiotrifluoroacetylacetonato)nickel-(II) showed great analytical potential from its spectrophotometric data, and the present paper describes a new and sensitive method for the determination of nickel with monothiotrifluoroacetylacetone which is superior to most of the recognised methods for this metal.