Singles and doubles coupled-cluster theory (CCSD) and the CCSD method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), are used to investigate the structure, vibrational frequencies, dipole moments, and relative stabilities of the XOCl and XClO (X = H, F, and Cl) isomers. It is shown that there is a large degree of ionic character involved in the bonding of the simple hypervalent chlorine compounds and that as the electronegativity of the X moiety increases, so does the stability of the XClO species relative to the XOCl isomer. Accurate relative energies are obtained at the CCSD(T) level of theory utilizing large atomic natural orbital basis sets, and these show that HClO and ClClO are less stable than the respective HOCl and ClOCl isomers, but that FClO is actually more stable than the FOCl isomer, indicating that F-Cl hypervalent bonding is more pronounced than Cl-Cl hypervalent bonding.