Abstract: In this paper some basic constants of extractant di(2-ethylhexyl)-phosphinic acid (P229) were determined. The solubility (S) of P229 in water was determined by the conductometric and pH methods. The ionization constant (K_α) of P229 in aqueous solution, distribution constant (K_d) and dissociation constant (K_αe) of P229 between octanol-water two phases were determined by two phases titration method. The results are as follows. 5=4.64×10^-5mol·dm^-3, pK_α= 4.58, pK_αe= 8.50, lgK_d=3.92 (25±0.2℃).