The devitrification behaviour of the glasses K2O·xGeO2 with x=4, 7 or 8 was examined by means of differential thermal analysis (DTA), the Fourier transformation infrared (FTIR) transmittance spectra and X-ray diffraction (XRD). The glass transition temperatures were related to the molar ratio GeO4/GeO6. For the glass with x=4, metastable K4Ge9O20> crystals are initially formed and then converted at higher temperatures into stable K2Ge4O9 crystals. The glasses with x=7 or 8 both devitrify into K2Ge7O15> crystals. The effects of the specific surface area of the samples on the devitrification mechanisms were established. Bulk nucleation predominates in the glass with x=4, while the glasses with x=7 or 8 crystallize from the surface. The activation energies for crystal growth were evaluated from the DTA curves.