Three pathways leading from B4H10 + C2H4 to (CH2CH2)B4H8 are examined at the [MP4/6-311+G(d,p)]//MP2/6-31G(d) + ZPC level. The preferred, "addition" path involves transient [B4H8], [B4H8(C2H4)], and a cage-opened [(CH2CH2)B4H8] intermediate with a rate-determining barrier of 33.0 kcal/mol (for [B4H8] formation). "Indirect hydroboration" of ethene via [B4H8] is disfavored by entropy and can be excluded. "Direct hydroboration" via a higher-energy [B4H10] isomer can be competitive; the overall energetic barrier for this process is only 26.9 kcal/mol, but the hydroboration step is disfavored by entropy to such an extent that the "addition" path is the main route at the experimental temperatures. Direct hydroboration affords the intermediate Et-B4H9 which is likely to be involved in side reactions leading to byproducts such as Et-(CH2CH2)B4H7.