Ab initio methods are used for investigating structural and electronic properties of silicon-carbon nanotubes (SiCNTs). Tubes with different Si to C ratios were tested and the trend from C-rich to Si-rich SiCNTs is examined. Our results show that SiCNTs lose stability when the ratio of Si over C increases. Nevertheless they remain stable until the ratio reaches 50:50, and after that the Si-rich tubes collapse to nanowires or clusters with solid interiors. The electronic density of states of different SiCNTs is also presented and analyzed.