A reliable conversion factor for the infrared absorptiometry of oxygen in silicon has been determined by round‐robin infrared measurement followed by charged particle activation analysis with the reaction. As for the round‐robin samples, 70 dislocation‐free CZ silicon wafers with oxygen contents ranging from and thicknesses of 2, 1, and 0.5 mm were carefully prepared by five organizations. A good linear relationship has been obtained between the absorption coefficient and the oxygen content. The relationship is expressed as .