The Hammett acidity functions (H0) of mixtures of Brønsted acids-ionic liquids were measured using a 13C NMR probe and several UV-Vis probes. Successful measurements for all of the systems were demonstrated using the UV-Vis probes. A suitable probe was chosen and used to measure each mixture with the estimation of H0 values from the 13C NMR probe. The 13C NMR probe can be used to calculate the acidity of the mixtures approximately while the UV-Vis probe gave more precise results. The H0 of Brønsted acids-ionic liquids systems with different acids were successfully measured using the 13C NMR probe. Additional measurements were done using the UV-Vis probe to get a full acidity measurements for each system that cannot be measured using the 13C NMR probe. Finally, the effect on acidity of the cation of the ionic liquid was found to depend on the strength of the acid mixed with ionic liquids. There is no difference in the acidity of mixtures with a weak acid.