Experimental and published data about lignin solubilization during the soda-anthraquinone pulping of a softwood mixture were successfully described with the aid of a generalized severity parameter K . R-OH. Based on a first-order decay process and a time-dependent rate constant, the K . R-OH concept combines the main operation variables (temperature, time and chemical loads) into a single reaction ordinate which is then used to summarize the changes in chemical composition and physical properties of the resulting pulp. The product K . R-OH proved to be a valuable tool which provides an absolute scale that enables different biomass species treated in different conditions to be compared.