A series of imidazolium-based room-temperature ionic liquids (RTILs) containing anions from organic carboxylic acids. were prepared. A set of dye probes, including Reichardt's dye (30), 4-nitrioaniline, and N,N-diethyl-4-nitroaniline, were used to determine the ET(30) scales and the Kamlet-Taft parameters (pi*, alpha, and beta) of the RTILs. On the basis of the polarity properties, these RTILs were categorized into three groups: group A with beta > 0.9, alpha < 0.9; group B with beta < 0.9, alpha < 0.9; and group C with beta < 0.9, alpha > 0.9. Interactions of these RTILs with four photochromic spiropyran derivatives (SP-I, SP-II, SP-III, and SP-IV) were investigated. It was found that the spiropyrans could present photochromism (positive or negative) or not, depending mainly on the polarity properties of the RTILs and also on the structure itself. A new spectroscopic method based on the molecular transition energy of the spiropyran probes (E-SP) was proposed to determine the polarity of those protic or fluorine-containing RTILs, which were failed with the Reichardt's dye (30) probe.