The extraction of tin through smelting of cassiterite (SnO 2) has generated considerable amounts of slag in the main production areas of SouthEast Asia, Africa, and South America. Tantalum and niobium naturally occur in cassiterite deposits and are concentrated in the slag during the smelting process. Tin slags have often been disposed as industrial waste despite contents of tantalum and niobium ranging from 2 to 25 %. Conventional hydrometallurgical processes for the concentration of tantalum and niobium oxides utilizing high concentration of strong acids such as HF or H 2 SO 4 are environmentally and economically challenging. This paper describes an original and greener approach for the selective dissolution of the slag amorphous matrix, and the concentration of tantalum and niobium oxides in a solid residue. Starting from a slag containing about 15 % (Ta,Nb) 2 O 5 , sequential alkaline and acid leaching stages resulted in the production of a 63 % (Ta + Nb) oxides concentrate, which is comparable to commercial grade of mineral concentrates.