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This paper presents the results from expansion tests on concrete prisms and mortar bars containing reactive aggregate and different types and levels of fly ash. Eighteen fly ashes representing those commercially available in North America were tested. The results show that the bulk chemical composition of the fly ash provides a reasonable indication of its performance in physical expansion tests but cannot be used to accurately predict the degree of expansion or the minimum safe level of fly ash required to suppress expansion to an acceptable limit. Generally, for a given fly ash replacement level (RL), the expansion increases as the calcium or alkali content of the ash increases or its silica content decreases. A corollary to this is that the minimum level of fly ash required to limit the expansion to an acceptable level increases as the calcium or alkali content of the ash increases or its silica content decreases. Most of the variation in fly ash performance can be explained on the basis of pore solution composition; those ashes effective in reducing the alkalinity of the pore solution extracted from cement paste samples were also efficient in controlling expansion. The data from this study provide further support for the use of the accelerated mortar bar test as a means for evaluating the efficacy of pozzolans in controlling expansion due to alkali–silica reaction (ASR).