Cu-Tl-Se compounds rank among the interesting structures used for electronic and opto-electronic applications. Therefore these synthetic systems are well studied. Beside great similarities with these systems we know that the natural minerals were formed by very complex substitution mechanisms leading to a great variety of solid solutions. The stability relations of the natural solid solutions exceeds in many cases the stability of the synthetic phases. A better understanding of the substitution mechanisms is fundamental for the synthesis of 'Cu-Tl-Se-x compounds' with enhanced features.