Laboratory investigations of the self-hardening properties of selected subbituminous fly ashes have been conducted. Chemical analyses of the fly ashes are given in Table I. The self-hardening value of the fly ashes was determined by conducting unconfined compressive strength tests on compacted samples of the moistened ashes. Various physical and chemical tests were performed to identify the reaction products, if any, of the hardened compacted fly ash paste, and to delineate the source of self-hardening properties. Results from x-ray diffraction analyses, scanning electron microscopic examination and differential thermal analyses indicated that the hydration products include calcium silicate and aluminate hydrates, and ettringite. Chemical and physical tests conducted to evaluate pertinent fly ash properties included chemical analysis, water soluble fraction, dilute hydrochloric and hydrofluoric acid soluble fractions, heat of solution on dissolving in dilute hydrochloric acid, specific surface area, and electrical conductivity tests.