The stability of sunset yellow FCF (El 10) was investigated in a model soft drinks system, with and without the addition of sodium metabisulphite. The rate of dye degradation was monitored by spectrophotometry and HPLC. In samples containing both sunset yellow and sodium metabisulphite, the formation of a lemon‐yellow‐coloured compound was clearly demonstrated. Isolation and subsequent analysis by NMR and FAB‐MS showed the structure of the compound to be that resulting from sulphonation at carbon 4 of the sunset yellow molecule. Examination of a stored commercial soft drinks sample indicated the presence of the same lemon‐yellow compound.