In a series of investigations on normal tetrahedral compounds we present mixed crystals in the System Cu2MnxCo1-xGeS4 (0 < x < 1) and an inspection of their tetrahedra volumes. Cu2Co-GeS4 crystallizes tetragonal in a stannite type structure, Cu2Mn-GeS4 crystallizes orthorhombic in the wurtzstannite structure type. The crystal structures Of Cu2CoGeS4 and Cu2Mn0.68Co0.32GeS4 were refined from single crystal diffraction data. The refinement of Cu2CoGeS4 converged to R = 0.0547 and wR2 = 0.0847 for 299 unique reflections. The refinement Of Cu2Mn0.68Co0.32GeS4 converged to R = 0.0481 and wR2 = 0.0877 for 1556 unique reflections. From these data the tetrahedra volumes of the end members and Of Cu2Mn0.68Co0.32GeS4 are calculated. In Cu2CoGeS4 tetrahedra [MS4] are similar in size. In contrast, the differences of the volumes of the polyhedra [MS4] in the orthorhombic wurtzite superstructure type compounds Cu2MnGeS4 and Cu2Mn0.68Co0.32GeS4 are significant (M = Cu, Mn, (Mn0.68Co0.32), Co, Ge). From x = 0 to x = 0.5 the tetragonal structure type dominates while from x = 0.7 to the Cu2MnGeS4 end member the products crystallize in the orthorhombic structure type. Melting points of the mixed crystals decrease linearly with increasing manganese content.