A loosely focused light beam is very useful for separating colloidal particles. For micron size colloidal particles suspended in a fluid and irradiated with a laser beam perpendicular to the direction of fluid flow, particles have a retention distance determined by their size and composition, the flow rate of fluid, and characteristics of the laser. An optical differential mobility analyzer for separating colloidal particles using a loosely focused beam is proposed theoretically and the concept of optical mobility is introduced. For the proposed optical differential mobility analyzer, particle trajectories and retention behavior are discussed and a transfer function described by operational parameters is derived. The possibility of separating particles of various sizes and different chemical compositions is discussed. It is proposed that the analyzer can be integrated into a microfluidic lab-on-a-chip system suitable for separating colloidal particles and biological cells at a very high resolution. (C) 2007 Elsevier Inc. All rights reserved.