A study of the Hþ-acetylacetone (acac, HC) and Hþ–V3þ-acetylacetone systems using emf(H) and UV–Vis measurements in 3.0M KCl ionic medium at 25 C is presented here. As the initial case, the acetylacetone pKa value was determined under similar experimental conditions as reported a few years ago, finding very similar values. Subsequently, in the study of the Hþ–V3þ- acetylacetone system, the stability constants of [VC]2þ, [VC2]þ, VC3, [OHVC]þ, OHVC2, [(OH)2V2C]3þ and [(OH)2V2C2]2þ complexes were determined by emf(H) measurements, and by means of UV–Vis measurements the respective values of [VC]2þ, [VC2]þ, VC3 and [OHVC]þ species, which are the most abundant in this system. The data analysis was carried out using the NERNST, FONDO, and SPEFO versions of the least-squares program LETAGROP. Some extra-thermodynamic relationships about the stability order of the VII, VIII, and VIV-acetylacetone complexes were obtained by ab initio calculations.