The European Union's reference method for the determination of a common fiscal marker of gas oil for heating purposes, Solvent Yellow 124 (SY124), was validated. A total of 12 different batches of samples using various commercially available gas oils and various colorants with SY124 concentrations from 0.12 to 9 mg L-1 were prepared. Various other dyes were added to check for potential interferences in the separation and detection of SY124. A total of 26 laboratories participated in the validation study. Outliers were identified using the Cochran and Hawkins test, and the resulting datasets were checked for normal distribution using normal probability plots. At a level of 6 mg L-1, the relative repeatability and reproducibility standard deviations were 0.68% and 3.8%, respectively. At 0.12 mg L-1, the repeatability and reproducibility standard deviations were 5.4% and 13.5%. A limit of detection of 0.020 mg L-1 and a limit of quantification of 0.065 mg L-1 were estimated. The method was found to be without significant bias and is therefore suitable as a reference method for the determination of SY124 in gas oil.