Mixed dissociation constant(s) of two nitrogen-containing bisphosphonate acids H(J)L, alendronate and ibandronate, at various ionic strengths I in the range of (0.02 to 0.30) Mat (298.15 and 310.15) K have been determined with the use of regression analysis of potentiometric titration data when common parameters PK(a,j), J = 1, ... , J, and group parameters L(0) and H(T) are simultaneously refined. Ibandronate and alendronate are the antiresorptive drugs most widely used to treat osteoporosis owing to their particularly high potency at inhibiting osteoclast-mediated bone resorption. External calibration of the glass electrode cell in the activity scale a(H+), was used. The estimate of ill-conditioned group parameters has a great influence on a systematic error in the estimated pK(a), and therefore it makes the computational strategy important. As more group parameters are refined and a better fit is achieved, a more reliable estimate of dissociation constants results. The thermodynamic dissociation constant pK(a)(T) was estimated by a nonlinear regression of {pK(a), I) data and a Debye-Huckel equation. A goodness-of-fit test based on regression diagnostics is the measure of a reliability of parameters and proves that reliable estimates for alendronate pK(a2)(T) = 2.60(1) and 2.76(1), pK(a3)(T) = 6.73(1) and 6.77(1), pK(a4)(T) = 11.51(2) and 11.29(1), and pK(a5)(T) = 12.44(3) and 11.82(3) at (298.15 and 310.15) K, respectively, and for ibandronate pK(a2)(T) = 2.33(1) and 2.50(1), pK(a3)(T) = 6.31(1) and 6.37(1), and pK(a4)(T) = 10.74(1) and 10.65(1) at (298.15 and 310.15) K, respectively, were found.