A new and effective organocatalytic system, primary amine derived chiral thiourea catalyst and AcOH/H2O additive, which converts different ketones to gamma-nitroketones in high yields (82-99%) and enantioselectivities (90-99%) is described. The transition state geometries for formation of R and S enantiomers in this Michael addition have been calculated and analyzed. It is shown that only one oxygen atom of the nitro group is bound to the thiourea moiety, in juxtaposition to the literature-known working hypothesis which involves a bonding of both oxygens. (c) 2006 Elsevier B.V. All rights reserved.