There has been significant utilization of standard fly ashes, Class C and Class F fly ashes, in the cement industry and in soil stabilization applications. However, the majority of the fly ashes that do not satisfy the chemical standards [off-specification (off-spec) fly ashes] have been disposed in landfills because of their unfavorable chemical properties such as high carbon content or high SO3 content. In this study, a clay soil was stabilized with different proportions of the standard and off-spec fly ashes. A series of unconfined compression (UC) tests were conducted on the specimens that were prepared to observe the suitability of the off-spec fly ashes for soil stabilization. The test results indicated that the off-spec fly ashes showed superior performance compared with the standard fly ashes due to their high CaO contents regardless of their SiO2+Al2O3+Fe2O3 and SO3 contents. It was recommended that chemical- and performance-based standards should be used instead of only chemical-based standards to determine whether a selected fly ash is suitable for soil stabilization.