The operational state of a surface inspection system for detecting defects on the surface of semiconductor wafers is monitored by: providing a reference wafer having defects of a particular number, size, and density on an examination surface; conducting a reference inspection of the reference wafer and at least one control inspection of the reference wafer by the surface inspection system, the position and size of defects on the examination surface being measured; identifying defects which, because of their position, are regarded as common defects of the reference inspection and of the control inspection; for each common defect, determining a size difference obtained from comparing its size from the reference inspection and from the control inspection; and assessing the operational state of the surface inspection system on the basis of the size differences.