Using a set of twelve selected substituents, the influence of substituent properties on the ease of deprotonation of imidazolium cations and mesoionic imidazolium-4-olates measured by the CREF index has been investigated. Significant correlations between CREF values and the Swain and Lupton field (F) and resonance (R) substituent constants have been found. In all cases the field effect has the greatest influence but resonance effects are also significant.