Process intensification is based on the idea that the transport rates are dramatically influenced when chemical processes are carried out in smaller path lengths and also in a continuous mode of operation. In this work, a special type of coating flow that leads to a thin film of liquid onto the inner surface of a horizontally rotating cylinder is studied. This type of flow is typically called rimming flow. At low angular velocities, the flow settles into a steady flow with a front where the coating film coalesces with the pool at the bottom of the cylinder. At higher angular velocities, a nearly uniform thin film (annular flow) is formed inside the rotating cylinder. There is extensive experimental as well as theoretical data available in literature on rimming flow of viscous liquids in partially-filled (no outflow) rotating cylinders. The aim of this work is to study rimming flow in a rotating cylinder that has a continuous outflow of liquid. Film thickness measurements (in the annular flow regime) are important to predict the transport performance of the developed flow system. Rimming flow of water is studied in this work and the film thickness measurements are done using an interferometric technique. The evaluation of the measurement technique and the experimental film thickness measurements will be presented. A correlation using dimensional analysis and also some preliminary theoretical work will be presented.