Composting of municipal solid waste with a 1 % addition of pulverized metallic iron, iron oxide(III) and iron sulfide(II) has been carried out. The amounts of iron in the bioavailable forms have been assayed in the composts obtained by means of speciation analysis, and the influence of composting on iron mobility has been evaluated. It has been found that pulverized metallic iron introduced into the waste occurs in the compost in the fractions easily accessible to plants, mainly the carbonate fraction. In the waste contaminated with Fe203 iron remains in the residual fraction, and composting does not practically increase its mobility. Over half of the iron from FeS remains in the waste in the residual fraction however, after composting there was an increased iron concentration in the bioavailable carbonate fraction.