Combined use of rotational spectroscopy and ab initio calculations at the MP2/6-311 ++G(d,p) level of theory shows that the complex formed between vinyl fluoride and hydrogen fluoride has a planar geometry in which HF forms a hydrogen bond to the F atom of vinyl fluoride. With phi = the angle CF... H = 121.4degrees assumed from the ab initio calculation, the distance r(FH) = 1.892(14) Angstrom and the angular deviation theta = 18.7(15)degrees of the (FH)-H-...-F nuclei from collinearity were obtained by fitting the principal moments of inertia of the two isotoporners (C2H3FHF)-H-... and (C2H3FDF)-D-... investigated. (C) 2004 Elsevier B.V. All rights reserved.