Miscanthus x giganteus stalks were subjected to delignification with formic acid-hydrogen peroxide-water in two stages. A face-centered experimental design Was used to study the effects of different variables (formic acid and hydrogen peroxide concentrations, time and temperature) on yield, lignin content and viscosity of pulps on the first stage. Fitting equations described satisfactorily the system behaviour and showed that hydrogen peroxide concentration and temperature were the most influencing variables. A set of independent variables (90% formic acid, 1.5% hydrogen peroxide, 60 min, and 67 degrees C) produced a pulp with low lignin content (kappa 17.2) and high viscosity (899 cm(3)/g), in the first stage. For the second stage a treatment time of 30 min was the more suitable, producing good quality pulps (kappa number 4.6, viscosity 788 cm(3)/g). Phenolic hydroxyl and carboxyl group contents were higher lignins from the first stage. (C) 2009 Elsevier Ltd. All rights reserved.