The initial phase of a bisulfite cook, here defined as the first part of the cook down to 20% lignin, is highly influenced by the temperature and it can therefore be used to control the initial pulping rate with respect to lignin. However, the influence of the temperature is different for the degradation of lignin, hemicellulose and cellulose and for the formation of the by-product thiosulfate. The temperature is therefore a powerful tool to control several mechanisms in a bisulfite cook. Additionally, if the cooking acid is taken from a pulp mill, where the ionic strength, the content of by-products and COD in the liquor phase are much higher than in a lab prepared cooking acid, this will also influence the kinetics of the different chemical reactions and, thus, the composition of the pulp after a certain cooking time. Earlier literature references, which are based on lab prepared cooking acids, are therefore not completely reliable as tools to predict the final composition of the pulp in a pulp mill.