Abstract: In the causticization part of the sulphate process the conductivity of the green liquor and the white liquor is measured, and the specific gravity of the green liquor is measured to determine the TTA-value (total titratable alkaline). The measuring values are registered on-line in a data processor calculating the required change of the supply of quicklime or green liquor to the slaker and/or the required change of the TTA-value of the green liquor for the control of the properties of the white liquor.The measurings carried out provide the basis of a control of the properties or composition of the white liquor, whereby the causticization process forming part of the sulphate process is simplified and made more efficient. The drawbacks associated with an insufficient control, e.g. calcification and the consequent poorer filtratability of the white liquor as well as a too high content of calcium in the white liquor, cause calcification of filters, pipes, pumps, and digesters etc.