The dissociations of alpha-hydroxy-N-nitrosopyrrolidine, 1, and alpha-hydroxy-N-nitrosopiperidine, 2, to form the nitrosiminium ions, 3 and 4, respectively, have been studied as models for the S(N)1 process of the corresponding alpha-acetoxy-N-nitrosamines. In excellent agreement with experiments, the ionization of the pyrrolidine derivative is found to be more endothermic at nb initio levels up to MP4(SDTQ)/6-31G*//RHF/6-31G* and including corrections for thermal motions. This finding is explained with more efficient N-->NO pi-dative bonding in 1 compared to 2 while ring size effects in 3 and 4 are rather small. The activation barriers to NN-rotation provide quantitative measures of the strengths of the N-->NO pi-dative bonding in 1-4 and the transition-state structures 1-TS-4-TS for the rotation about the NN bonds in 1-4 were determined, Electron density analyses (NBO) and electrostatic field analysis (CHELPG, MKS) were performed to assess the extent of N-->NO pi-dative bonding in alpha-hydroxy-N-nitrosamines and the electronic relaxation associated with the competition between amine N-->NO and N-->carbocation pi-dative bonding in N-nitrosiminium ions. The comparative analysis of the structural and electronic relaxation associated with ionization in the presence (1-4) or absence (1-TS-4-TS) of the possibility for N-->NO pi-dative bonding demonstrate in a compelling fashion that sigma-polarizations are responsible for most of the electron density relaxation.