Chemical activation of pine sawdust using aqueous solutions of phosphoric and boric acid was studied. Thermogravimetric analyses of the impregnated lignocellulosic precursor as well as their reactivities are presented. The role of the activating agents in the thermal decomposition of the precursor and their effects on the development of porosity in activated carbons is discussed. The use of boric acid as activating agent and a final heat treatment temperature (HTT) of 600degreesC produced some porosity and activated carbons with a surface area of 600 m(2) g(-1). With a HTT of 450degreesC, molecular sieve-like materials were produced. In contrast, phosphoric acid activation produced activated carbons with high surface areas. The amount of phosphoric acid retained in the precursor was, as expected, a function of the initial concentration as well as the impregnation time and temperature. The fact that the activated carbon porosity increases with the phosphoric acid content per gram of precursor was confirmed. The use of 6M phosphoric acid solution and HTT of 450degreesC produced the activated carbon with the highest surface area, about 1600 m(2) g(-1). The high degree of microporosity which developed in phosphoric acid-activated carbons is related to the low reactivity during the thermal treatment of chemical activation. The presence of up to 10% oxygen during HTT with phosphoric acid activation caused only small textural changes. (C) 2003 Society of Chemical Industry.