The effects of exchange and impregnation of zeolite ZSM-5 catalysts with alk.-earth metals (Mg, Ca, and Ba) and some transition metals (e.g., Ag, Cu, Cr Zn, Mi, Ce, Co, and Mn) on the conversion of MeCl were studied. The activity, selectivity, and lifetime of the catalysts significantly depended on the cation chosen for modification. Temp.-programmed desorption of NH3 and BET measurements showed that acidic properties as well as free-channel passages are affected by exchange and impregnation. Mg-exchanged ZSM-5 exhibited both the highest activity and the lowest deactivation due to lower formation of coke and aroms., esp. C9+ aroms. This was confirmed by a long-term aging test with Mg ZSM-5 pellets under semi-tech. conditions. Correlations were derived between certain product selectivities and electronegativity equalization concepts of both Pauling and Sanderson.