Transition structures for the addition of ammonia to s-cis and s-trans acrolein, methyl vinyl ketone, and 2-butenal have been located with ab initio molecular orbital calculations. Optimization of the geometries of reactants, transition structures, and products for the conjugate and carbonyl addition mechanisms were done at the restricted Hartree-Fock level with 6-31 + G* basis sets, and electron correlation energy was calculated at the MP2 level with a 6-31 + G** basis set. The relative reaction barriers for conjugate and carbonyl additions were discussed for gas and solution phase reactions. Calculations predicted that the reaction barrier for conjugate addition was lower than that for carbonyl addition. Conjugate addition is an exothermic process at all levels of theory employed. The conjugate addition products are also more stable than the carbonyl addition products. (C) 2001 Elsevier Science B.V. All rights reserved.