An electron‐beam probe has been developed to measure the potential profile of a plasma within a spherical grid structure which was designed to trap charged particles. The potential was obtained from observed deflections of a collimated beam of electrons incident upon the device at selected impact parameters. The beam consisted of conversion electrons of 62.2 and 84.2 keV from a Cd109 source. Deflections of the beam through the plasma were measured relative to the beam trajectory without voltage applied to the grids. A movable detector allowed angular resolution to within 0.08 deg for maximum deflections of 7 deg. The inversion of the nonlinear singular integral relating potential and deflection is accomplished in an iterative manner with appropriate series expansions about the singular points. Simulations using postulated potential profiles show this procedure to give accuracy typically better than 4% with 20 deflection samples. Beam deflection measurements in spherical electrostatic systems without plasma show this probe technique to be very accurate. For a variety of experimentally controlled plasma parameters, no measurable departures from electrostatic potential profiles were observed.