Two reference materials with certified mass fractions of SY124 in gas oil have been prepared. Samples were prepared by the spiking of blank gas oil with pure SY124. Homogeneity and stability were confirmed, and maximum heterogeneity and degradation were estimated. The purity of the SY124 used for spiking was determined using thermogravimetric analysis, high-performance liquid chromatography with UV detection, gas chromatography with flame ionization and mass spectrometric detection, nuclear magnetic resonance, and Karl Fischer titration. Full uncertainty budgets comprising all potential uncertainty sources were established. The following mass fractions were derived: ERM-EF317, 0.141 +/- 0.018 mg kg(-1); ERM-EF318, 7.0 +/- 0.4 mg kg(-1).