The critical properties, the normal boiling temperatures, and the acentric factors of ionic liquids have been determined using an extended group contribution method based on the well-known concepts of Lydersen and of Joback and Reid. The critical properties of ionic liquids cannot be experimentally determined in many cases since most of these compounds start to decompose as the temperature approaches the normal boiling point. However, for the development of thermodynamic models either for pure components or for mixtures containing ionic liquids, the critical properties and other physical parameters are required. The so-called group contribution methods have been commonly used to estimate the critical properties of many substances for which these properties are not available, but no attempt has been made to estimate the critical properties of ionic liquids, as presented in this study. The method does not require any additional data besides the knowledge of the structure of the molecule and its molecular weight. Since experimental critical properties of ionic liquids are not available, the accuracy of the method is checked by calculating the liquid densities of the ionic liquids considered in the study. The results show that the values determined for the critical properties, for the normal boiling temperatures, and for the acentric factors are accurate enough for engineering calculations, for generalized correlations, and for equation of state methods, among other applications.