The pozzolanic reactivity of a commercial metakaolin (MK) manufactured in Brazil, was evaluated in pastes prepared with a type II Portland cement, when used in cement substitution degrees from 10 to 40 mass % by thermal analysis techniques. The reactivity during the early stages of hydration was studied in real time from Non-Conventional Differential Thermal Analysis curves, normalized by the effect of the specific heats of the raw materials. The total pozzolanic reactivity after 28 days of hydration was evaluated by the calcium hydroxide consumption, calculated from thermogravimetric and derivative thermogravimetric curves of the pastes, obtained on cement oxide calcined basis. This also allows one to a more accurate determination of the uncombined and total combined water in the pastes than usual procedures. The MK pozzolanic reactivity during the early stages and after a month is proportional to the cement substitution degree in the pastes.