The induction periods of BaSO4 nucleation from supersaturated solutions have been measured over a wide range of conditions in the presence and absence of phosphino-polycarboxylic acid (PPCA). The inhibition of Barite nucleation in the presence of PPCA was correlated to the solution chemistry over a wide range of conditions. Only the dissociated fraction and the metal-complexed fraction of PPCA are effective to inhibit BaSO4 nucleation, while the protonated fraction has a poor inhibitory effect toward Barite nucleation. Ca2+ enhances the overall efficiency of PPCA through forming Ca-PPCA complexes and decreasing the fraction of protonated PPCA. Temperature does not strongly affect the Barite nucleation inhibition by PPCA. A set of equations has been proposed to model Barite nucleation time in the presence and absence of PPCA.