The agricultural residues of Crete are considered to be of premium importance for local energy production, substituting a large part of conventional fuels. Future co-firing applications of local lignite and agroresidues may create problems related to the disposal of ashes, which contain harmful constituents. In this regard, the mobility of trace elements from selected lignite and biomass ashes, as well as mixtures of them, was investigated through batch and sequential extraction procedures. In these tests, leaching conditions were aimed at a close approximation to field conditions. The concentrations of heavy metals released in local soil of acidic nature were determined by atomic absorption spectroscopy and were compared to those in the original samples. The results demonstrated that toxic metal ions were released in low quantities, below the legislative limit values, with the exception of Cr extracted from specific lignite/biomass mixtures. This leaching behavior was attributed to the incorporation of the trace elements in the crystal lattice of the soil minerals. A small portion was also captured by carbonates and/or iron oxides of the soil or the ash residue. Biomass ash addition to the ignites' ashes was beneficial, reducing the heavy metal contents of the soil leachates.