The hydrolytic species of lanthanide ions, La3+ and Sm3+, in water at I = 0.1 mol center dot dm(-3) KCl ionic strength and temperatures of 298.15, 310.15 and 318.15 K were investigated by potentiometry. The hydrolytic species were modeled by the HySS simulation program. From the results, the hydrolytic species of each metal ion at different temperatures were calculated using the program HYPERQUAD2013. The hydrolysis constants (log(10) beta) of [La(OH)](2+) and La(OH)(3) were calculated as -8.52 +/- 0.46, -26.84 +/- 0.48, and log(10) beta values of [Sm(OH)](2+), [Sm(OH)(2)](+), Sm(OH)(3) were calculated as -7.11 +/- 0.21, -15.84 +/- 0.25 and -23.44 +/- 0.52 in aqueous media at 298.15 K, respectively. The dependence of the hydrolysis constants on the temperature allowed us to calculate the enthalpy, entropy, and Gibbs energy of hydrolysis values of each species.