Fenton's process has proven to be a viable alternative for the oxidation of organic pollutants in mixed waste chemicals, with a high efficiency in the removal of heavy metals from the effluent as a side effect. However, a solid residue loaded with metals is obtained as a by-product. In this paper, the solids originating from the Fenton's oxidation of mixed waste chemicals from an academic laboratory were studied. The process was applied to different wastewater cases (two composite samples generated in different periods) in order to study the effect of their characteristics on the residues composition. Residues were evaluated and classified according to their leaching properties. In addition, a most comprehensive assessment of metal mobility in the residues was obtained by chemical speciation. The Fenton's residues were classified as hazardous according to Brazilian waste regulations. The application of the sequential dissolution procedure indicated that the metals in the Fenton's residues are mainly constituted of amorphous material (over 80%). Furthermore, the reactive fractions of the residues (exchangeable and amorphous iron oxide fractions) retain most of remaining metals. Therefore, the Fenton's residues present great potential for environmental contamination, and require an administration system and control of their final disposal.