The production of silicon carbide and silicon nitride whiskers in silica/carbon solid mixtures under nitrogen-containing atmospheres was investigated. The morphology of the products was analysed by scanning electron microscopy and its composition was determined by elemental analysis. The effect of hydrogen in the atmosphere was studied, and it was concluded that the presence of this gas lowers the temperature necessary for SiC formation and increases the SiO2/C reaction yields. The introduction of transition metals in the solid mixtures allowed SiC and Si3N4 whisker formation by a vapour-liquid-solid mechanism, and the morphological characteristics of the materials produced can be controlled by the use of other additives, like alkaline and alkaline-earth metals and rare earths.