Fly ash adsorption capacity is a critical property for the beneficial utilization of fly ash; although this capacity could be favorable for some applications, it may not favor the use of fly ash for other applications such as cement and concrete production. In both cases the lack of fly ash adsorption capacity quantification tools and procedures limits the beneficial utilization of fly ash. This study compared the results of the currently available fly ash adsorption capacity tests to a newly developed test for the direct measurement of the adsorption capacity of fly ash. The study showed, as previous work did, that the commonly used thermogravimetric method, called loss on ignition (LOI), is not a good measure for quantifying the adsorption capacity of fly ash, the foam index test was more successful as an indicator rather than a measurement of the adsorption capacity, and the fly ash iodine number successfully quantified the adsorption capacity of fly ash and correlated very well with the direct measurement of the air entraining admixture (AEA) adsorption by fly ash. In addition, this study showed that the relationship between the fly ash iodine number and the AEA adsorption capacity measured via the direct adsorption isotherms can be used to determine the quantity of AEA adsorbed by the fly ash using the fly ash iodine number of the fly ash of interest. This AEA quantity represents the AEA dosage adjustment required to compensate for the adsorption of the AEA by fly ash in concrete.