A pH sensitive scanning probe is realized by integrating a micron-sized field-effect sensor onto a cantilever designed for an atomic force microscope. The hybrid device, called a scanning probe potentiometer (SPP), is capable of measuring pH gradients over a sample surface. The device was used to profile the pH across a reservoir of laminar streams created by fluid flow in an array of microfluidic channels of varying pH. When a single SPP scanned, a 1.5 mm reservoir in a 10-channel array, the pH profile was measured in less than 1 min with a spatial resolution of 10 μm and sensitivity of less than 0.01 pH units. © 2000 American Institute of Physics.