The effect of protonation on the conformation of N,N,N',N'-tetramethylethylenediamine (TMEDA) has been studied by Raman spectroscopy and ab initio theory. TMEDA and TMEDAH(2)(2+) molecules assume predominantly the irons NC-CN conformation in aqueous solution, while a targe number of TMEDAH(+) molecules assume the gauche NC-CN conformation, as stabilized by strong intramolecular 1,4-N-H+. . .N hydrogen bending.