The presence of FeCl(2) catalyst enabled the growth of one-dimensional nanostructures directly during the pyrolysis of highly porous monoliths, produced from a polysiloxane preceramic polymer with the aid of a gas-generating porogen. Either silicon nitride or silicon carbide nanowires were formed, with a length of several micrometers, depending on the processing atmosphere. Increasing the pyrolysis temperature caused an increase in the length and the amount of nanostructures produced. The remaining matrix consisted of an incompletely crystallized Si-O-C phase, containing SiC crystals and either graphitic (N(2) pyrolysis) or amorphous carbon (Ar pyrolysis). X-ray diffraction data and high-resolution transmission electron microscopy investigations combined with electron energy loss and energy-dispersive X-ray spectroscopy methods enabled to ascertain the growth mechanisms for the nanowires, which depended on the pyrolysis atmosphere (gas phase reaction for N(2) pyrolysis; vapor-liquid-solid for Ar pyrolysis).