Pulp mills producing hardwood and softwood pulp in campaigns have constantly changing process stream compositions. There is aneed to know how these variations develop during the campaigns and that was the objective of this study. A dynamic model, built in the simulation program WinGEMS, has been created by including tanks and reactors with known residence times. A case with a mill producing softwood and hardwood pulp in campaigns has been designed and studied. Some of the studied parameters were white liquor concentration, recovery boiler load and composition of weak black liquor. The dynamic changes of these parameters were not unexpected; however, the time responses and magnitude of these changes were of bigger interest. For instance, it took a little more than 3 days, from the start of a new campaign, to totally replace the weak black liquor from one wood species with the other. In order to interpret results successfully, it was necessary to be aware of the residence time between certain positions in the pulp mill model.