MOS transistors and CMOS devices comprising a plurality of transistors including metal-based gate electrodes of different composition are formed by a process comprising: depositing a first blanket layer of a first metal on a thin gate insulator layer extending over first and second active device (e.g., a MOS transistor) precursor regions of a semiconductor substrate; selectively forming at least one masking layer segment on the first blanket layer overlying selective ones of the MOS transistor precursor regions; depositing a second blanket layer of a second metal or semi-metal, or silicon, over the thus-formed structure; effecting alloying or silicidation reaction between contacting portions of the first and second blanket layers overlying the other ones of the transistor precursor regions; exposing and selectively removing the masking layer segment; and simultaneously patterning the alloyed and unalloyed/unsilicided portions of the first blanket layer to form metal-based gate electrodes of different composition. The invention also includes MOS and CMOS devices comprising differently composed metal-based gate electrodes.