In the mixed crystal system CdxSn1-x Te the transition from a ten (x = 0) to an eight-electron system (x = 1) is realized with a simultaneous change from rocksalt (SnTe) to zinc blende structure (CdTe). From previous calculations using density functional theory (DFT) [in the local density approximation (LDA) together with normconserving pseudopotentials and the virtual crystal approximation (VCA)] we know, that the rocksalt structure is stable up to x = 0.93. As a possible reason for this stability we investigate the valence charge transfer connected with the formation of the solid by employing the concept of zero-flux surfaces, which has been used so far only for molecules. We find an increasing charge transfer when going form CdTe to SnTe in the rocksalt structure. In addition we analyze the bonding character of the individual valence bands and find partial antibonding for CdTe in zinc blende. Both support the stability of the rocksalt structure, which is found experimentally.