This paper presents an experiment suitable for validating calculated pH gradients inside an occluded cell. Photolithography is used to pattern a 10 mu m thick cavity former on a stainless steel surface, providing a rigorously controlled experimental support for transport models such as those proposed in the fields of crevice corrosion or cathodic delamination. pH gradients over the electrode are measured by total internal reflection fluorescence microscopy (TIRFM) during cathodic polarization under potentiostatic and galvanostatic conditions. The pH profiles calculated using a pseudo-two-dimensional (pseudo-2D) transport model agree well with experimental data, except that the rates of pH increase observed experimentally are much slower than the simulated ones.