Conformational analysis of poly(ethylene imine-alt-ethylene sulfide) (PEIES) has been carried out from ab initio molecular orbital calculations and NMR experiments for its model compound, N-(2-methylthioethyl)methylamine (MTEMA). The meso-diad probability, bond conformations, and characteristic ratio of PEIES were predicted by the refined inversional-rotational isomeric state (IRIS) calculations using conformational energies determined from MTEMA. The refined IRIS scheme can treat geometrical parameters and interaction energies as a function of conformations of the present and neighboring bonds. The configurational properties of poly(ethylene imine-alt-ethylene oxide) (PEIEO), poly(N-methylethylene imine-alt-ethylene oxide), and poly(ethylene imine) (PEI) were also evaluated by the refined IRIS computations and compared with those obtained by the conventional method. So long as geometrical parameters are properly chosen, the conventional IRIS method may yield results comparable to those by the refined scheme. Spatial configurations of these polymers depend chiefly on intramolecular attractions between the heteroatoms; the characteristic ratio (< r(2)>(0)/nl(2)) inversely correlates with the strength of N-H center dot center dot center dot X (X = O, N, or S) attraction: PEIEO with N-H center dot center dot center dot O of -1.75 kcal mol(-1), < r2 >(0)/nl(2) = 1.33; PEI with N-H center dot center dot center dot N of -1.54 kcal mol(-1), 3.09; PEIES with N-H center dot center dot center dot S of -0.97 kcal mol(-1), 5.15.