Chemistry Department, Faculty of Science, Aswan, Egypt Manuscript received 23 September 1987, revised 11July 1988, accepted 14 December 1988 The electronic absorption spectra of 4,5-dioxo-1-phenylpyrazoline com- pounds were recorded In organic solvents of different polarities, in mixtures of solvents and in buffer solutions. The longer wavelength band is assigned to intermolecular CT Interaction. The spectra recorded in the CT complex region in mixed solvents revealed the formation of molecu'ar complexes between the solute and EtOH, DMF, DMSO. The stability cons- tant and heat of formation of these complexes have been determined. The basicity constants of the compounds are calculated and discuss