The extreme cathodic and anodic pH fronts induced in the electrochemical treatment of tumors (EChT) are the main cause of tumor necrosis. Here, we study pH fronts interaction in a tissue under EChT through in vitro and in silico modeling. The in vitro model considers the tumor tissue as a buffered gel with saline (NaCl) content and the cathodic treated area as the area of phenolphtalein virage. The in silico model solves the 1D Nernst-Planck equations for ion transport in a four-ion electrolyte. In silico modeling predicts an initial neutral pH profile evolving into extreme cathodic alkaline and anodic acidic fronts moving towards each other and colliding, thus excluding the existence of a biological pH region between them. Moreover, the model predicts that as electric current increases, pH front scaling grows as t(1), unveiling a transition from a diffusion to a migration governed regime. Theory and simulations have a strong correlation with experimental measurements. Since necrotic areas correlate well with those covered by alkaline and acid fronts advance, pH front tracking can be used to predict the extent of tumor destruction and thus, the assessment of EChT effectiveness. (C) 2009 Elsevier Ltd. All rights reserved.