Six b-dipole rotational transitions have been observed between 8 and 9.2 GHz for the transient species 1,1-dimethylsilaethylene (DMSE) produced by thermal decomposition of 1,1-dimethylsilacyclobutane at ~1000 °C. The Si=C and C-C bond lengths determined from these observations are in excellent agreement with theoretical predictions [Acc. Chem. Res. 1982, 15, 283]. Analysis of the quartet fine structure from internal rotation gives a barrier V3 = 1004 (17) cal/mol, a methyl top moment of inertia Iα = 3.18 (5) µÅ2, and a top axis to b-axis angle θ = 55.6 (1)°. The fit of the line centers with a rigid-rotor model finds A, B, and C rotational constants to be 6037.93, 5896.78, and 3093.70 MHz, respectively. The heavy-atom effective structure determined from these parameters is ro(Si=C) = 1.692 (3) Å, ro(C-Si) = 1.868 (3) Å, and ∠(C-Si-C) = 111.4 (2)°. The methyl groups do not appear to be tilted appreciably; however, the observed ∠(C-Si-C) is 2.3° smaller than predicted. The furnaces and procedures developed for the formation and observation of DMSE are described. They are applicable to other transient and reactive species.