A colorimetric absorption test for evaluating fly ash/air-entraining admixture (AEA) behaviour with regard to their use in concrete was investigated. Acid blue 80 (AB80) standard dye sorbate (typically used to examine activated carbon) was considered. Initial experiments established a calibration between the concentration of AB80 in solution and absorbance; and a suitable test procedure to determine the AB80 adsorption (i.e. difference in initial and final concentrations of AB80 solution after exposure to fly ash). In general, a 2.0 g fly ash sample in 100 ml of 100 mg/l AB80 and a contact time of 30 min were satisfactory test conditions. Factors including dye adsorption during filtration and the effect of high/low adsorption fly ashes were examined and methods for dealing with these described. To evaluate the test, 15 fly ashes were considered and their AB80 adsorptions compared with several markers of fly ash/AEA behaviour. This gave reasonable agreement with loss-on-ignition, and very good correlations with specific surface area (measured by N adsorption) and foam index. In general, 1 mg of AB80 was adsorbed per m surface area of fly ash. The study also investigated relationships between AB80 adsorption and admixture doses required (using several AEAs) to achieve target air contents in mortar and concrete, which gave very good correlations. The practical implications of the study were considered and it is suggested that the AB80 method has potential both as a test for characterising fly ash and in air-entrained concrete production with the material.