The ability of silica to form different phases can be used to stabilize its nanostructures. Here we explore the stability of thin nanotubes of SiOx (x=1 and 2) using ab initio calculations with the generalized gradient approximation for the exchange-correlation energy. We find that the pentagonal nanotubes are energetically most stable. The pentagonal SiO nanotube is a semiconductor with the largest calculated band gap of 0.90 eV, which is close to the value for bulk Si. The SiO2 nanotubes are, however, insulating similar to bulk silica and could be promising as the thinnest insulating layers for nanodevices. Our results demonstrate that we can get the most important circuit elements for nanoelectronics, namely semiconducting, as well as insulating nanotubes based on silicon in the subnanometer regime. {\textcopyright} 2005 The American Physical Society.