Transmission Mössbauer Spectroscopy (TMS) and Conversion Electron Mössbauer Spectroscopy (CEMS), together with X-ray diffraction (XRD) are applied to identify the origin of stain defects formed on one-side zinc-coated steel used in the automotive industry. The stained sheets were directly exposed to CEMS meanwhile absorbers for TMS were obtained by tape, scrapping, paper and ultrasound bath. Measurements, both by TMS and CEMS, show the presence of the α-Fe signal in all the cases. CEMS measurements also indicate that a Mössbauer inactive coating covers the stained areas. Fe₃C and Fe-C signals, as minor ones, appear only when the absorber is prepared by paper. Results show that the stains are not due to a corrosion process and therefore the corrosion performance of the steel should not be affected by the presence of the stains over the non plated side. An incomplete cleaning of the oil used to protect the steel before entering the electrogalvanizing line would have produced the stains.