This study was conducted to determine iron (Fe) contamination on supposedly clean laboratory glassware used for Fe determinations. Iron adhered tenaciously to glassware that had been well washed and rinsed by usual methods (detergent, HCl, and distilled water) and was removed only by washing the glassware with a Fe che‐lating agent. Variability sometimes noted in Fe determinations could possibly be attributed to Fe contamination on glassware. Our data showed that glassware previously exposed to Fe solutions should be washed with an appropriate Fe chelating agent before subsequent determinations of Fe.