A new colorimetric method based on the bleaching of the iodoplatinate ion has been developed for fast and easy determination of γ-glutamyl-S-ethenyl-cysteine (GEC) in narbon vetch (Vicia narbonensis L.) seeds. The calibration curve showed a good correlation (r(2)=0.9959) between absorbance and GEC amounts from 5.5 to 33 μg (10-59.78 μmol/L). The limits of detection and quantification were 1.16 and 3.55 μmol/L, respectively, and no significant interferences from other sulfur-containing compounds were observed. The method showed excellent repeatability (relative standard deviation [RSD]=0.28%), reproducibility (RSD=4.4%), and accuracy (94%). Determination of GEC in 20 narbon vetch accessions yielded values that were in agreement with those reported previously using capillary electrophoresis and high-performance liquid chromatography methods. The method could be especially valuable for determination of GEC during the process of production of new low-GEC narbon vetch varieties.