Predictions of a mathematical model for oxygen delignification are compared with measurements made at three mills having system designs and operating conditions covering those used industrially. Pulp samples were taken for analysis throughout each system, and most importantly, immediately following mixing. Data were compared with model predictions for the kappa number (lignin content) and viscosity (pulp strength) using system operating variables during the test. Two methods are compared for modelling changes in pulp strength: the first ratios the extent of depolymerization of cellulose (DP) to that of delignification, while the second ratios DP loss to the extent of reaction with glucose. Measurements show that a significant extent of the total delignification and pulp viscosity loss occur during the brief mixing process. The accuracy to which the model can predict this depends on the assumed composition of the entering lignin.