The hot-dip galvanizing sludges have been studied with the aim of establishing phase relations and chemical processes taking place during the thermal treatment. Phase relations in the sludge heated for 1-5 h between 100 and 1000 degreesC were studied by infrared, powder X-ray, Mossbauer, and scanning electron microscopy methods. Dehydration and decomposition processes were also monitored by a thermogravimetry- mass spectrometry technique. Results of our investigations indicate that during the thermal treatment ZnFe2O4 and ZnO were formed, while alpha- and gamma-FeOOH (goethite) as well as the basic zinc chlorides were decomposed and finally cannot be detected in the 1000 degreesC heat-treated samples. Because the major part of the zinc is present in the form of aqueous ammonia-insoluble ferrite, instead of ammoniacal leaching, another technique using, e.g., iron(II) sulfate, has to be used for recovering the zinc from these types of heat-treated samples.