Overcalcined clays could be available as industrial by-products of the ceramic or refractory industries or may be produced as a result of an uncontrolled clay calcination process. Therefore, it is important to understand the effect of overcalcination on the behavior of calcined clays as pozzolans. In this paper, the pozzolanic reactivity of a kaolinitic clay with a high kaolinite content and calcined at temperatures from 800 to 1000 ºC is investigated. Analysis by XRD and XRF indicates a 72% kaolinite content and an abundant content of iron oxides and hydroxides, mostly associated with amorphous or poorly crystalline phases. Pozzolanic reactivity is evaluated by compressive strength in mortars and by alkaline solubility of Al and Si. Sieving analysis shows that a finer particle size distribution is obtained for the clays calcined at higher temperatures under the same grinding protocol, suggesting that calcined clay grindability increases with calcination temperature. Although pozzolanic reactivity decreases with calcination temperature, the total alkaline solubility of potentially reactive species for the clay calcined at 1000 ºC is still 60% as compared to the material calcined at 800 ºC. Both compressive strength and alkaline solubility of Al and Si follow the temperature sequence: 800 ºC > 900 ºC > 1000 ºC. For the series containing 30% substitution of OPC by the calcination products at 1000 ºC, the compressive strength at 28 days is similar to the control series, which could be interpreted as the combined result of a filler effect and a slow development of the pozzolanic reaction.