New data are presented on drop size distribution at high dispersed phase fractions of organic-in-water mixtures, obtained with a light back scattering technique (3 Dimensional Optical Reflectance Measurement technique, 3D ORM). The 3D ORM technique, which provides fast, in-situ and on-line drop distribution measurements even at high concentrations of the dispersed phase, is validated using an endoscope attached to a high-speed video recorder. The two techniques compared favourably when used in a dispersion of oil (density (p) = 828 kg m(-3), viscosity (mu) = 5.5 mPa s, interfacial tension (sigma(i)) = 44.7 mNm(-1)) in water for a range of 5-10% dispersed phase fractions. Data obtained with the ORM instrument for dispersed phase fractions up to 60% and impeller speeds 3 50 - 5 50 rpm showed a decrease in the maximum and the Sauter mean drop diameters with increasing impeller speed. Phase fractions did not seem to significantly affect drop size. Both techniques showed that drop size distributions could be fitted by the log-normal distribution. (c) 2005 Society of Chemical Industry