Jet-cooled laser-induced fluorescence spectra of the (B) over tilde <-- (X) over tilde electronic transition of 1-methylvinoxy and of a cis/trans mixture of 2-methylvinoxy are presented. We observe some 50 vibronic bands in the spectrum of 1-methylvinoxy. The complexity is due to a change in the preferred methyl rotor orientation on electronic excitation. The (B) over tilde-state barrier to internal rotation is approximately 750 cm(-1). The (B) over tilde-state fluorescence lifetimes decrease from about 130 ns near the origin to 26 ns for internal energy of 2700 cm(-1). We observe some 22 vibronic bands in the spectrum of the cis/trans mixture of 2-methylvinoxy. The (B) over tilde-state fluorescence lifetimes decrease gradually from about 190 ns near the origin to 140 ns for internal energy of 1170 cm(-1) and then very sharply at higher energy. The new spectra serve as fingerprints for identification of specific methylvinoxy isomers as products of chemical reactions of O(P-3) with alkenes, as recently observed by Bersohn and co-workers.