Proton abstraction from acetaldehyde by the first- and second-row hydride anions NH2-, OH-, F-, SiH3-, PH2-, and SH- have been investigated with ab initio quantum mechanical calculations up to the MP2/6-31+G**//HF/6-31+G* and MP2/6-311+G**//MP2/6-31+G* level. Progress of energies, geometric variables, and charge distributions along the reaction coordinate have been investigated by employing the intrinsic reaction coordinate method. These computations revealed that proton abstractions occur in three essentially separate steps. The main difference between the first- and second-row hydrides is that the carbon, which donates proton in the reaction, changes hybridization from sp(3) to sp(2) later in the reaction coordinate in the case of the first-row systems. Consequently, the flow of negative charge from the CH2 to CHO end of the developing enolate anion is retarded more in the case of the first-row hydrides. The results showed that the proton abstractions by the first-row hydride anions have more imbalanced transition states than the second-row hydride anions.