Spiro-polyenes, compds. with two perpendicular p-systems joined by a common spiro-atom, exhibit through-space spiroconjugation. One important representative of spiroconjugated systems is spiro[4.4]nonatetraene (3). The mol. structures of spiro[4.4]nona-1,3,7-triene (2) and (3) were detd. by x-ray diffraction and compared with the redetd. structure of cyclopentadiene (1) to detect the slight distortions expected from spiroconjugation. Crystallog. data are given. Comparison of bond lengths and angles reveals a slight shortening of the double bonds and a small elongation of the single bonds connecting the spiro atom in (3). The same effect is also found from ab initio calcns. at Hartree-Fock level 6-31G*, although to a minor extent. This is shown by applying a difference anal. of structural parameters to cancel out the systematic errors and differences of both methods.