Deliquescence points (RH0, RH0mix) of ionic crystalline food ingredients and blends thereof were determined using water activity and moisture sorption techniques. Measured RH0mix values of ingredient blends with and without a common ion were compared to Ross equation predictions of deliquescence lowering. In binary blends with no common ion, measured RH0mix values ranged from 5% RH lower to 6% RH higher than predicted; however, when a common ion was present, the measured RH0mix was consistently 6-8% RH higher than predicted. In tertiary blends with a common ion, RH0mix values were 15-18% RH higher than predicted. The higher RH0mix in blends with a common ion is caused by counterion dissociation competition from the common ion. At equilibrium, these solutions will have fewer solutes and a greater vapor pressure than assumed by the Ross equation. A modified Ross equation was developed to compensate for the common-ion effect and improve RH0mix predictions.