Mining residues management is one of the greatest challenges for mining companies around the world. The increasing consciousness of the general public and governments about the potential threat that those residues can pose to the environment is demanding consistent and precise methodologies for assessing the potential release of toxic metals. On this regard, the modified BCR (R) sequential extraction procedure (SEP) is frequently the chosen assessing protocol. However, this protocol was designed to study soils and sediments with low to moderate metal pollution, and validation of its applicability to mining residues is missing. The present research covers this gap of knowledge by subjecting selected highly polluted mining residues to the modified BCR (R) SEP. On the light of these results, it was confirmed that most of the metal bearing minerals in the mining residues were not completely dissolved in the corresponding SEP and, therefore, the application of BCR (R) SEP to mining residues systematically leads to an underestimation of metals mobility. The necessary changes to optimize the BCR (R) SEP to study mining residues would set a extraction procedure distinctively different from the original; thus it is strongly recommended to use alternative approaches to assess toxic metals mobility in highly polluted mining residues.