Calibration with an internal standard was applied to eliminate particle size effects for powdered samples of copper ores, concentrates and copper slag prepared in the form of pellets. Internal standards were added in a powder form of extremely small particle size, which means that changes in their particle size could cause only negligible changes in their x‐ray fluorescence intensity. The observed clear relationship between the internal standard x‐ray fluorescence intensity and sample particle size was applied for the correction of x‐ray fluorescence intensities of many analytes. Copyright © 2002 John Wiley & Sons, Ltd.