The methodology and experiment for certification of reference specimens for determining interstitial oxygen concentration in semiconductor silicon are reported. These reference specimens are intended for calibration of infrared spectrophotometers which measure the 1107 cm(-1) oxygen peak in silicon to enable users to improve their measurement agreement. Based on an earlier international Grand Round Robin study, this measurement agreement is at best 5.4% (2 sigma). Industry requirements for measurement comparison are much more demanding, and a methodology to satisfy those requirements is described. The most important aspect of this methodology is to reduce interlaboratory variation by the use of a single infrared instrument for certification. The certification uncertainty depends primarily on the improved repeatability of this instrument. Other sources of uncertainty were nonuniformity in both oxygen concentration and thickness over the specimen area, and variations in residual oxygen among the float-zone specimens which provided zero-oxygen reference for the reference sets. These various sources were combined in quadrature to arrive at 2 sigma estimates of uncertainty under 0.2% at three oxygen levels.