Removal of residual water droplets from produced fluids of petroleum is a challenging task because the emulsified water droplets with size of a few micrometers are extremely stable. A focused beam reflectance measurement (FBRM) was used for on-line monitoring of emulsified water droplet size distribution in petroleum emulsion systems and chemical demulsification process. After establishing the feasibility of using FBRM to measure droplet size distributions in petroleum emulsions, factors influencing the average size of water droplets were investigated to evaluate the demulsification dynamics by ethyl cellulose demulsifiers. The FBRM is proven to be a valuable tool for monitoring demulsification process in situ in real time to understand demulsification mechanisms, which sheds the lights on improving the current technology of demulsifying water-in-oil emulsions of petroleum industry. (C) 2015 Elsevier Ltd. All rights reserved.