Conformational characteristics of poly(ethylene imine) (PEI) have been investigated by a rotational isomeric state (RIS) analysis of ab initio molecular orbital (MO) calculations and H-1 and C-13 NMR experiments for a monomeric model compound, NIT-dimethylethylenediamine (di-MEDA). From the MO and NMR data, it was shown that the C-C and C-N bonds of di-MEDA have high gauche (71-93%) and trans (64-86%) preferences, respectively. Conformational energies of PEI were determined from the MO calculations for di-MEDA at the MP2/6-311++G(3df, 3pd)/HF/6-31G(d) level. The high gauche stability in the C-C bond was indicated to stem from a moderate and a weak intramolecular N-(HN)-N-... hydrogen bonds; the interaction energies were evaluated as -1.54 and -0.58 kcal mol(-1), respectively. The RIS scheme including rotational and inversional isomerizations was developed and applied to PEI to evaluate the chain dimension and diad probabilities. With the conformational energies determined as above, the characteristic ratio and meso-diad probability of PEI at 25 degreesC were calculated to be 2.9 and 0.63, respectively. In polar and protic solvents, the intramolecular hydrogen bonds are weakened, and consequently the PEI chain extends. Branching effects on the conformation were investigated. from MO and NMR analysis for monomeric model compounds of branched PEI, N,N,N'-trimethylethylenediamine and N,N,N',N'-tetramethylethylenediamine; the gauche preference in the C-C bonds, due to the hydrogen bonds, is reduced with increasing number of methyl groups. Ab initio MO calculations were carried out for the double-stranded helix found in anhydrous PEI crystal. The PEI chain was indicated to adopt the isotactic form exclusively. The natural bond orbital analysis showed that intermolecular N-(HN)-N-... hydrogen bonds are formed between paired chains of the double helix. The enthalpy of association per repeating unit was estimated to be -3.6 kcal mol(-1) at the MP2/6-311+G(2d,p)//HF/6-31G(d) level.