The mechanisms of nitrosation of acetone through sodium enolate [CH₃COCH₂]⁻Na⁺ (1) or naked enolate [CH₃COCH₂]⁻ (2) with tert-butyl nitrite (CH₃)₃CONO (3) were studied using ab initio molecular orbital (MO) methods. When the modified complex model was used in the elimination process, our results demonstrated the predominant formation of E-1-hydroxy-imino-2-oxo-propane CH₃COCH=NOH (4E), in which a counter-cation of the base catalyst did not participate during the reaction. On the other hand, participation of the counter-cation during the reaction contributed to the formation of the Z-isomer of 4 (4Z).