Sulphuric acid is an important raw material in the chemical industry, used in many different processes. Oleum is produced by dissolving SO3 in H2SO4. Sulphuric acid is produced by dissolving water in oleum (also called double contact process). The most important thing with sulphuric acid is that it becomes oleum above 100%. This makes it a very interesting liquid. In order to monitor the dilution process, an accurate and reliable real-time in-line analyser is needed. Rhosonics has developed a very accurate analyser for this purpose, namely model 8500. This ultrasonic analyser is able to determine concentrations in different possible ranges, based on the velocity of sound and through measurement of the liquid’s temperature. The concentration of aqueous solutions of sulphuric acid or oleum is reported as %w/w H2SO4.