We study the effects of Si doping on the bonding nature and elastic properties of AlMB14 compounds having B12 icosahedral clusters with M Li, Mg and Na, by means of ab initio calculations. We find that Si substitution leads to a decrease in the cohesive energy as well as in the values of the bulk, shear and Young's moduli. {\textcopyright} 2009 IOP Publishing Ltd.