The S₁-S₀ electronic spectrum of methyl cinnamate in a supersonic jet has been investigated to discriminate the transitions of the s-cis/s-trans conformers. Population labelling spectroscopy was applied to the conformer discrimination, and vibronic bands belonging to each conformer were identified. The relative population was estimated fromo the fluorescence quantum yields obtained by lifetime measurements. It was found that both conformers exist almost equally in a supersonic jet. The conformer identification of the vibronic bands was carried out based on the difference of the red-shift of their hydrogen-bonded complexes with methanol.