Lithium nitrate is recognized as a chemical compound capable of inhibiting the alkali-silica aggregate reaction. The extent to which the reaction is inhibited or limited is affected by the amount of the applied lithium compound and by the kind of reactive aggregate that has been used. In the case of the polymineral aggregate this process is particularly complex, as the alkali-silicate reaction may be accompanied by other alkali-aggregate reactions, i.a. the alkali-carbonate reaction. The paper presents the results of the investigation on the effect of lithium nitrate on the course of the alkali - reactive gravel aggregate reaction. The investigation was conducted in accordance with ASTM 1260 standard. The investigation results showed that lithium nitrate, when applied in appropriate amount, inhibits the process of mortar samples expansion. The appearance of spalls in the samples modified with a lithium compound indicates the occurrence of two reaction types: alkalisilica and alkali-carbonate.