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Fly ash, a solid waste generated from coal burning, can replace up to 70% of the cement in concrete and reduce environmental impact, cost of construction, and power generation. However, the lack of fly ash adsorption capacity measurement tools limits this type of fly ash beneficial utilization. This article presents the combined adsorption isotherms as a tool for the accurate measurement of the adsorption capacity of fly ash in concrete. The combined adsorption isotherms are simpler yet more accurate than the separate adsorption isotherms for quantifying the adsorption of air entraining admixtures (AEAs) by fly ash in concrete. Instead of performing two separate isotherms, one for cement and another for fly ash, enough mass of cement is added to all fly ash isotherm points to achieve full chemisorption. Blank isotherm points, consisting of the AEA and cement, determine the initial concentrations of AEA available for adsorption. Any AEA concentration reduction caused by the various masses of fly ash in the remainder isotherm points is considered physical adsorption. The difference between the two AEA concentrations is used to determine the adsorption capacity of the fly ash. This approach reduces the fly ash adsorption isotherm test procedure to one isotherm and provides a more simplified form of testing. It also eliminates the need to use judgment in determining the AEA partitioning coefficient from a multipoint cement isotherm. This paper presents the test development and application on Vinsol resin admixture with eight fly ashes. The results obtained from these isotherms were used to make AEA dose adjustment predictions to compensate for the AEA adsorption onto the fly ash for cement mortars and concrete with 25% fly ash replacement. All dose adjustments succeeded in producing the target air void content in cement mortars and concrete mixes.