A new empirical formula, based on the surface fractal dimension, 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 pyrolysed in a 600-900 degrees C temperature range. The following equation is proposed: Y (wt%) = root D-f/ln epsilon[L[0.59 - 2.74 X 10(-4)(t degrees C-600] + A - 0.1E[1 + 2 X 10(-3)(600 - t degrees C)]] where D-f, epsilon, L, C, A and E are parameters of the precursor, namely: fractal dimension, 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%. (c) 2005 Elsevier Ltd. All rights reserved.