The interactions between iron(III) acetylacetonate and a number of phosphine derivatives in methylene chloride were studied at 25-degrees-C by measuring the absorbances of the solutions at 430 nm. The absorbances were used to calculated equilibrium concentrations of the species in the solutions and then to evaluate equilibrium constants of these interactions. The equilibrium constant values indicate the following trend in the basic strength of the 1:1 adducts formed between phosphine derivatives and iron(III) acetylacetonate: (C2H5)3P>(ClC6H4)3P> (C6H5)3P>(C6H5)2(CH3O)P>C6H5(CH3)2P. The formation of the above adducts is also supported by NMR study.