Unlike chromate, cobalt occurring as cobalt oxides in cement is not water‐soluble in a detectable amount. Cobalt oxides are to some extent voluble in the presence of amino acids with which cobalt forms complexes. Such complexes can elicit patch test reactions. It is postulated that cobalt is more readily dissolved by forming complexes in eczematous skin than in normal skin. This may explain why cobalt sensitization in cement eczemas is secondary to chromate sensitivit