Ab initio molecular orbital theory at the MP2/6-311t-G(3df,2p)//B3-LYP/6-31G(d) level has been used to study the transition-metal-free catalysis of the hydrogenation of ethene. Catalysis by HX, (HX)(2) and HAlX4 (X = F, Cl, and Br) has been examined. Both concerted pathways and stepwise pathways involving CH3CH2X-type intermediates have been characterized. The former are energetically preferred in the case of the HX-and (HX)(2)-catalyzed reactions. However, for the HAlX4-catalyzed hydrogenations, concerted and stepwise mechanisms are found to have similar barriers, The HAlX4 species are found to be very effective hydrogenation catalysts, reducing the barrier for the hydrogenation of ethene from the value of 367 kT mol(-1) in the uncatalyzed process to less than 100 kT mol(-1) for all the halogens (X).