Results of an international comparison of iodine absorption cells organized by the BIPM are reported. Ten of the thirteen member laboratories of the Comité Consultatif pour la Définition du Mètre (CCDM) participated. Two experimental methods were used to estimate the purity of the iodine in the cells: first, the cells were incorporated in BIPM He-Ne lasers and the laser frequencies thus obtained at λ = 633 nm were compared by the beat frequency method with those of the BIPM reference lasers and, second, direct measurements of iodine purity by a laser-induced fluorescence method were made at λ = 502 nm. Of the thirty-nine cells investigated, more than half are pure enough to realize a frequency reproducibility to within 10 kHz (2 parts in 1011 in relative terms). A contamination of 129I2 has been observed in certain cells containing 127I2. The comparison confirms that observed frequency shifts of the hyperfine components in iodine are strongly linked to impurities in the iodine cells.

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