The distribution of ash present in the various lignocellulosic material components (extractives, holocellulose, cellulose and lignin) obtained during chemical analysis has been studied for several lignocellulosics with ash contents higher than 4%. The summative mass balance of the ash-free fractions was compared with the initial amount of ash-free raw material. Untreated lignocellulosics were studied and so were substrates autohydrolyzed at two temperatures, 180 and 210 degrees C, for 15 min of isothermal residence time. Results showed that the different fractions have to be ash corrected to prevent their yield being overestimated. It was also observed that hydrolytic treatment did not change the ash distribution among the different fractions when compared with the untreated lignocellulosics. Ash tends to remain attached to the solid fraction after the fractionation sequence. This results in overestimating the total yield since the ash fraction is accounted for several times. A procedure for correcting the yield of each fraction and the mass balance closure is proposed here. This procedure is based on determining the weight of each fraction on an ash-free basis, while the total ash content is added to close the balance. (C) 1997 Elsevier Science Ltd.