The scanning laser microscope (Par-tec) has been used to measure drop size distributions of oil-water mixtures in both batch and on-line operations and the performance of the instrument has been assessed. Measurements have been made to determine the influence of the shear produced by pipes and fittings on the dispersion characteristics of oil-water mixtures. The influence of oil composition has also been investigated. The Par-tec has been found to give reproducible mean chord data for oil-in-water and water-in-oil dispersions in both a batch mixing process and on-line at various dispersed phase concentrations and for both clear and optically dense oil systems. However, there were some important limitations noted. Firstly, the drop size measured by the instrument is not a drop diameter as measured by most other particle sizing instruments but a drop chord length; secondly, the drop size was found to vary with the focal length of the instrument. Changes in drop size due to changes in process conditions can easily detected using the Par-tec instrument. In this work the effect of horizontal pipe length and number of bends was investigated. It was found that the drop size increased with increasing horizontal pipe length due to drop coalescence. The drop size was also found to increase as the number of bends in the pipe was increased in a 1.5 I.D. U-pipe fitting, indicating that the energy dissipated in the system is not enough to outweigh the effect of coalescence due to the pipe length. The sequence of pipe fitting was found to influence the nature of dispersion. The value of the mean chord at the exit of a needle valve followed by a small U-pipe fitting was not the same as the mean chord for the U-pipe followed by the needle valve. The main advantages of this instrument are its flexibility (it can be easily inserted into process streams in most pipes and vessels) and its ability to obtain data in two phase systems with high dispersed phase concentrations and/or an optically dense continuous phase.