Experiments of delignification of Pinus pinaster wood by acetic acid at atmospheric pressure showed that acetic acid concentration is a primary factor in the process and that high concentrations (90%) significantly improve the delignification degree. The kinetics of the process were satisfactorily explained by a model of consecutive first-order reactions corresponding to lignin solubilization followed by lignin condensation and reprecipitation onto the lignocellulosic matrix, hydrolysis of alpha-aryl ether bonds being the dominant reaction of the delignification process. The selectivity of the lignin dissolution was practically independent of operating conditions. When the degree of delignification reached a value around 90% the selectivity decreased considerably due to lignin condensation. (C) 1997 Elsevier Science Ltd.