The jet-cooled laser induced fluorescence spectrum of the (B) over tilde <-- <(X)over tilde> electronic transition of the 1-methylvinoxy radical is assigned, including both hot and cold bands. The barrier to methyl internal rotation in both (X) over tilde and (B) over tilde states is determined by fitting pure torsional transitions to a one-dimensional hindered-rotor model. The resulting 3-fold torsional barrier parameters are V-3' = -740 +/- 30 cm(-1) for the (B) over tilde state (minimum-energy conformation with one methyl CH bond cis to the frame CO bond) and V-3" = +130 +/- 30 cm(-1) for the (X) over tilde state (methyl CH bond trans to CO). The intensity pattern clearly indicates a change in the preferred methyl conformation upon excitation, while ab initio calculations provide the absolute conformations in each state, A variety of ab initio methods including CASSCF, multireference CI, and coupled-cluster techniques were applied to both the (X) over tilde and the (B) over tilde states of 1-methylvinoxy. Only the largest coupled-cluster calculations yield a (B) over tilde -state barrier in good quantitative agreement with experiment. In unsubstituted vinery, a (B) over tilde -state geometry adjusted earlier to fit experimental rotational constants (ref 10) is evidently in error.