High surface potential stability has been realized by postannealed SiO2 electret thin films prepared on Si substrates using various deposition methods: r.f. magnetron sputtering, metalorganic chemical vapor deposition, utilizing electron cyclotron resonance plasma and vacuum arc plasma evaporation. It was found that the surface potential stability of SiO2 thin films prepared by these deposition methods could be improved by postannealing in an oxidizing atmosphere at a temperature of 350-400 degreesC. In addition, the obtained surface potential stability was also dependent on the deposition conditions. The surface potential of SiO2 electret thin films prepared under optimized deposition and postannealing conditions was found to exhibit no decay when tested for longer than 50 h at a relative humidity of 90% and a temperature of 60 degreesC. However, the mechanism causing the improvement in surface potential stability remains insufficiently understood.