A device has been designed to measure the pressure of the wafer-pad interfacial film during chemical mechanical polishing. Measurements have been collected and compared from trials where the wafer was held stationary and where it was rotated. Stationary trials resulted in an asymmetrical pressure distribution, a large pressure range, and some subatmospheric values. Dynamic trials showed less variability, no subatmospheric values, and pressure increasing monotonically from edge to center. Flow separation was observed under wafers with a concave shape and resulted in erratic pressure fluctuations.