The effects of C by N replacement on the intramolecular hydrogen bond of malonaldehyde have been analyzed through the use of B3LYP/6-311++G(3df,2p)//B3LYP/6-311+G(d,p) calculations. Formylformimidic acid (1) exhibits a IMHB much stronger than that of malonaldehyde, reflecting the enhancement of both the intrinsic acidity and basicity of the HB donor and HB acceptor, when C is replaced by nitrogen. Similar effects are observed for N-(aminomethylene) formamide (2), N-(iminomethyl)formimidic acid (3) and N-(aminomethylene)formimidamide (4). The corresponding saturated compounds present much weaker IMHBs, but this is clearly due to the characteristics of the C1σ-skeleton, which forces the HB donor and acceptor not to be coplanar and away from each other. As a matter of fact, the corresponding planar Cs structures exhibit a much stronger IMHBs than the C1 equilibrium structures. Quite unexpectedly, the IMHB length in nitrosoformaldehyde oxime (5) is slightly weaker than in malonaldehyde, due to the fact that the oxime is not planar in its equilibrium conformation, leading to an unfavorable orientation of the HB donor with respect to the HB acceptor. Again, if the symmetry of the system is constrained to be Cs a significant reinforcement of the IMHB is observed, ratifying that the main factor governing the strength of the IMHBs in these compounds the σ-skeleton and not the possible resonance effects.