Attempts to prepare [Se(CH 2 CH 2 COOH) 3 ] + Clfrom Se(CH 2 CH 2 COOH) 2 and H 2 C=CHCOOH in concentrated hydrochloric acid, for the corresponding sulfonium salt, led exclusively to the Se-betaine, Se(CH 2 CH 2 COOH) 2 (CH 2 CH 2 COO). The Se-betaine crystallises in the space group P2 l /c with the cell dimensions at 223 K, a = 5.5717(1), b = 24.6358(4), c = 8.4361(1) Å, β = 104.762(1)°, V = 1119.74(3) Å 3 , Z = 4, D calc = 1.763 Mgm-3 , µ = 3.364 Mm-1. The structure refined to R I = 0.0223 for 2801 reflections with F o > 4σ(F o). In the crystalline state the molecule is intermolecularly linked to neighbouring molecules by a number of hydrogen bonds; a very strong carboxylic-carboxylate bond with an O••••O distance of 2.4435(16) Å, a medium strong carboxylic-carboxylate bond with an O••••O distance of 2.6431(16) Å and several weak O••••H(CH 2) with O••••C distances between 3.2 and 3.3 Å. In the carboxylic group involved in the very strong hydrogen bond the O••••H bond is antiperiplanar to the C=O bond while the O-H bond is periplanar to the C=O bond in the second carboxylic group. Based upon the CO bond lengths and the elongation of the O-H bond involved in the strong hydrogen bond one may describe the compound as strongly linked units of Se(CH 2 CH 2 COOH)(CH 2 CH 2 COO) 2 rather than Se(CH 2 CH 2 COOH) 2 (CH 2 CH 2 COO). The selenium atom forms two strong intramolecular 1,5-Se••••O contacts, with a carboxylate oxygen atom, 2.9385(12) Å, and with a carboxylic oxygen atom, 2.8979(11) Å. To allow for these contacts the two organic fragments have been forced into the periplanar conformation. The molecule is only slightly asymmetric with regard to the C-Se-C bond angles but is very asymmetric with regard to the torsion angles.