Treatment of toluene solutions of the silylenes Si(SArMe6)(2) (Ar-Me6 = C6H3-2,6(C6H2-2,4,6-Me-3)(2), 1) or Si(SArPr4i)(2) (ArPr4i = C6H3-2,6(C6H3-2,6-Pr-2(i))(2), 2) with excess ethylene gas affords the siliranes ((ArS)-S-Me6)(2)SiCH2CH2 (3) or ((ArPr4S)-S-i)(2)SiCH2CH2 (4). Silirane 4 evolves ethylene spontaneously at room temperature in toluene solution. A Van't Hoff analysis by variable-temperature H-1 NMR spectroscopy showed that Delta G(assn) = -24.9(2.5) kJ mol(-1) for 4. A computational study of the reaction mechanism using a model silylene Si(SPh)(2) (Ph = C6H5) was in harmony with the Van't Hoff analysis, yielding Delta G(assn) = -24 kJ mol(-1) and an activation energy Delta G double dagger = 54 kJ mol(-1).