Abstract: A method and apparatus for measuring the acid gravity in an inorganic acid-organic fluid system permits constant monitoring of the acid gravity thus allowing the acid strength to be maintained at an optimum level. Particularly in a system such as an alkylation reactor, where product quality is dependent upon acid strength, the acid gravity measuring method and apparatus assures high product quality. A collecting vessel receives the inorganic acid-organic fluid mixture and the acid gravitates from the organic fluid in a disengaging chamber. The acid then enters an acid-analyzing section which includes acid gravity measuring means for determining the specific gravity of the separated acid. By continuously draining the fluid from the disengaging chamber and the acid-analyzing section, fresh fluid can be continuously introduced and the acid gravity continuously analyzed.