The structures of 4-hydroxybenzylidenemalononitrile (HO-C 6 H 4 -CH C(CN) 2 , I), its oxyanion ( - O-C 6 H 4 -CH C(CN) 2 , II), cyanide adduct (HO-C 6 H 4 -CH(CN)-C(CN) 2 , III) and adduct-oxyanion ( - O-C 6 H 4 -CH(CN)-C(CN) 2 , IV) have been studied by means of both quantitative IR spectra and ab initio force field calculations. The conversion of (I) into the anionic species causes strong changes in the IR spectra: decreases in the ν C N frequency down to 110cm - 1 , up to 7-fold increases in the A C N intensity, up to 58cm - 1 ν C N splitting, etc. The charge analysis shows that the intramolecular charge transfer between the electronegative [C(CN) 2 ] and electropositive fragments of (I) is 0.34e - . Nearly 0.6e - of the oxyanionic charge of (II) remains within the oxyphenylene fragment and nearly 0.5e - of the carbanionic charge of (III) delocalizes within the dicyanomethide fragment. The two charges in (IV) are spread over the whole species.