The high extracellular hydrogen peroxide (H₂O₂) concentrations generated during pharmacological ascorbate (P-AscH^-) therapy has been shown to exhibit a high flux into susceptible cancer cells leading to a decrease in clonogenic survival. It is hypothesized that the intracellular H₂O₂ concentration for susceptibility is independent of cell type and that the variation observed in dosing is associated with differences in the cell-specific overall steady-state intracellular H₂O₂ concentration values. The steady-state variation in intracellular H₂O₂ concentration is coupled to a number of cellular specific transport and reaction factors including catalase activity and membrane permeability. Here a lumped-parameter mathematical modeling approach, assuming a catalase-dominant peroxide removal mechanism, is used to calculate intracellular H₂O₂ concentration for several cell lines. Experimental measurements of critical parameters pertaining to the model are obtained. The cell lines investigated are normal pancreatic cells, H6c7, the pancreatic cancer cell line, MIA PaCa-2 and the glioblastoma cell lines, LN-229, T98G, and U-87; all which vary in susceptibility. The intracellular H₂O₂ concentration estimates are correlated with the clonogenic surviving fraction for each cell line, in-vitro. The results showed that, despite the fact that the experimental parameters including catalase concentration and plasma membrane permeability demonstrated significant variability across cell lines, the calculated steady-state intracellular to extracellular H₂O₂ concentration ratio did not vary significantly across cell lines. Thus, the calculated intracellular H₂O₂ concentration is not unique in characterizing susceptibility. These results imply that, although intracellular H₂O₂ concentration plays a key role in cellular susceptibility to P-AscH^- adjuvant therapy, its overall contribution in a unifying mechanism across cell types is complex.