Laboratory experiments show that pretreatment with NO2including heating and ripening at 5% consistency and 90°C for 3 hours in the presence of NO3- and H+led to a dissolution of approximately 50–60% of the lignin in unbleached kraft pulp. Only 5–15% were dissolved when the treatment was terminated after heating to 90°C. An extensive fragmentation of the lignin during the ripening explains the extensive delignification in acid medium.