This study presents an optimization of the method used to quantify reactive silica (RxSiO(2)) in gibbsitic bauxites, specifically those which contain no (or insignificant) quartz. In this work they are named as Paragominas-type Bauxites. A factorial design of experiments was used. The variables included the temperature, caustic concentration and time for two cases: bauxites with low and high RxSiO(2) contents. Temperature is the most important factor and has a positive influence on RxSiO(2). First-order empirical models were properly obtained to predict the amount of RxSiO(2) as a function of temperature, caustic concentration and time, which responded to the following optimal conditions: (1) without significant amount of quartz - 180 degrees C, NaOH 10% w/v and 60 min for low RxSiO(2) and 25 min for high RxSiO(2), and (2) with significant amount of quartz - 150 degrees C, NaOH 20% w/v and 60 min for both situations. (C) 2017 Published by Elsevier B.V.