Mercury photosensitized decomposition of trans-2,3-epoxybutane has been studied at 25°. The quenching efficiency of this compound is 1.23 relative to the quenching efficiency of N2O taken as unity; the quenching cross section is, therefore, 19 Å2 (taking 3.6 Å2 for the quenching cross section of n-butane). The main products formed are isobutanal, C2H6, CO, C2H4, C3H8, CH3CHO, C2H5CHO, CH3COC2H5, CH4, and H2. The yields of most of the products decrease with increasing pressure while those of C2H6, CH4, and H2 remain essentially pressure independent. Possible reaction mechanisms are discussed.