A new empirical formula, based on the porosity, the botanical composition and the ash content of the precursor, is proposed for the assessment of the char yield, Y (wt%), of lignocellulosic materials pyrolyzed in a 600 degrees C-900 degrees C temperature range. The following equation is proposed: Y (wt%) = (0.8815- 2.281/epsilon + 61.44/epsilon(2)/ center dot{[L(0.59-2.74 x 10(-4)t degrees C- 600)/+ 0.22C] + A - 0.1E[1+2 x 10(-3) (600 - t degrees C)]} where epsilon, L, C, A, and E are parameters of the precursor, namely: porosity (%) and percentage ( wt%) of lignin, cellulose, ash, and extractives, the t temperature, being in the Celsius grade. The values calculated by the formula fit satisfactorily with the experimental results, the gap being less than 0.5%.