A fully coupled thermo-hydro-mechano-chemical (THMC) formulation is described in this paper. Special attention is paid to phenomena likely to be encountered in clay barriers used as engineered barriers in the disposal of nuclear radioactive waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetics- and equilibrium-controlled reactions are incorporated. The formulation is implemented in a numerical code. An application is presented concerning the performance of a large-scale in-situ heating test simulating high-level radioactive waste repository conditions.