Vacuum pyrolyses of 1,1-dimethyl-1-germa-3-thietane (2) and 1,1,3,3-tetramethyl-1-germacyclobutane (3) most likely proceed with the formation of the same transient species, 1,1-dimethyl-1-germene (1), detected both in the gas phase by electron impact mass spectrometry (EIMS) and in argon solid matrixes at 12 K by Fourier transform infrared (FTIR) spectroscopy. The production of methylgermylene (6) in the reactions studied has also been suggested, probably as a result of secondary isomerization of 1 into methylethylgermylene (8) and subsequently to 1-methyl-1-germacyclopropane (9), and further dissociation of 9 at higher temperatures, in agreement with the theoretical consideration of the suggested mechanism. Full vibrational assignments for the IR spectra of 1 and 6 have been proposed on the basis of ab initio and density functional theory calculations of the harmonic vibrational frequencies and infrared intensities, and of Literature data on related-molecules. The assignment of a high-intensity IR band at;847.3 cm(-1) to a Ge=C stretching vibration coupled with the CH3 rocking mode in 1 has also been supported by the calculation of potential energy distribution in the internal coordinates and by the observed splitting of this band due to natural isotopic abundance. of germanium, being in accord with the B3LYP-calculated Ge-70/Ge-72/Ge-74/Ge-76 isotopic frequency shifts in 1. The calculated force constant for the Ge=C bond in 1 confirms a pi nature of this bond and yields an estimated bond order to be somewhat lower than the Si=C pi-bond order in 1,1-dimethyl-1-silene (4).