Abstract Effect of the Na+ ion concentration in the surface layer of alkali - treated Ti6Al4V alloy on the heterogeneous nucleation of calcium phosphate was verified in non-buffered precalcificated solution (SCS) at pH 6.2. The alkali-treated Ti6Al4V samples (c(NaOH) = 10 mol.l-1, 60 °C and 24 hrs.) were stored in aqueous solutions with different concentrations of Na+ ions (0; 0.1; 10 mol.l-1) resulting in dramatic changes in Na+ concentration in the surface layers. Substitution of Na+ by Ca2+ ions in SCS solution is rapid. Capacity of samples to be covered by crystals when immersed in SCS solutions (super - saturated by Ca2+ , PO43- ) decreases with diminishing Na + concentration in the hydrated layers of Ti6Al4V. Stability / capability of calcium phosphates nucleation (HA–hydroxyapatite / Ca10(PO4)6(OH)2; OCP–octacalcium phosphate / Ca8(HPO4)2(PO4)4.(5H2O); TCP–tricalcium phosphate / Ca3(PO4)2; a.o.) in SCS solution and in SBF solution (simulated body fluid) was assessed by thermodynamic driving forces. Phases HA, TCP a OCP are termodynamical stable in the solution SCS (c(ca2+ ) = 5 mmol.l.-1 and c(PO43- ) = 2,5 mmol.l.-1) by pH 6,2 .