An HF olefiri/isoparaffin alkylation process is carried out in an alkylation unit with a settling vessel in which the alkylate product is separated from the HF acid catalyst containing water and acid soluble oil (ASO). The density of the liquids in the settling vessel is measured at different levels by means of a nuclear density profile analyzer. The acid strength of the acid phase is determined from the density measurement and an optional temperature measurement. The proportion of water in the acid phase may also be measured separately by measurement of its electrical conductivity to determine the respective contributions of the water and the ASO to the density of the HF acid phase.
