The X-ray structures of four dioxolan-2-ylium cations, 2-phenyl-1,3-dioxolan-2-ylium trifluoromethanesulfonate, 6, and 2-(4-methoxyphenyl)-1,3-dioxolan-2-ylium, 7, 2-(2,4-pentadienyl)-1,3-dioxolan-2-ylium, 8, and 2-(4-methoxyphenyl)-4,4-dimethyl-1,3-dioxolan-2-ylium, 9, hexafluoroantimonates, have been determined. Crystals of 6 are monoclinic, C2/c, a = 28.934(5), b = 6.022(2), c = 14.831(3) Å, β = 113.37(1)°, Z = 8; 1944 reflections were used to determine the structure; R = 0.0488, wR = 0.0477. Crystals of 7 are monoclinic, P2 1 /c, a = 11.948(4), b = 10.686(3), c = 10.853(2) Å, β = 106.31(2)°, Z = 4; 2260 reflections were used to determine the structure; R = 0.0551, wR = 0.0807. Crystals of 8 are monoclinic, P2 1 /c, a = 10.549(2), b = 10.457(3), c = 11.702(2) Å, β = 105.69(1)°, Z = 4; 2190 reflections were used to determine the structure; R = 0.0337, wR = 0.0299. Crystals of 9 are orthorhombic, Pmnb, a = 8.297(4), b = 15.269(3), c = 12.638(2) Å, Z = 4; 1736 reflections were used to determine the structure; R = 0.0796, wR = 0.0726. Intensities were measured on Syntex P2 1 , or Nicolet P3 diffractometers at −65 °C with use of Mo Kα radiation. These structures are discussed in terms of the conjugation and charge distribution in these cations. In each case the cations are almost planar with very small twist angles between the plane of the C(2) substituent and that of the dioxolanylium ring. Cation 8 is the first reported example of the structure determination of an acyclic, substituted dienyl carbenium ion. The unsaturated five-carbon chain in 8 has an all-trans conformation about all the C—C bonds, but, apart from the C(2)—C(1′) distance, none of the other bond distances differ significantly from those found in corresponding neutral systems. The C(2)—C(1′) distance in 7 is significantly shorter than that of 6, suggesting enhanced conjugation of the aryl ring with the positive centre in the former cation. This suggestion is supported by the significant quinonoid-type distortion that is observed in the p-methoxyphenyl ring of 7. Systematic variations in the O(1)—C(5)/O(3)—C(4) bond distances are examined and it is shown that alkyl substitution at C(4)/C(5) leads to a large increase in their lengths, suggesting that the presence of a significant fraction of the positive charge is located on C(4) and (or) C(5). Support for this suggestion is found in a large variety of previously reported spectroscopic and calorimetric measurements.