A mathematical model is presented to compensate for the optical coincidence of particles in a liquid particle counter (LPC) detector, which detects particles by means of light scattering. It compensates for the optical coincidence of particles ( 1) belonging to the same size category, ( 2) belonging to different size categories, or ( 3) promoted from two particles to a single particle, which belongs to the larger particle size category. With this improved method, particle concentration decays are studied in a continuous stirred tank reactor where the liquid is recirculated and filtered. Furthermore, it is found that the latex sphere-calibrated LPC makes large errors in particle sizing, which is caused by the differences between the refractive index (n) of the particles under study: polystyrene latex spheres (0.112-0.204 mu m), silica ( 0.16 and 0.33 mu m), and Si(3)N(4) (< 0.4 mu m). This implies that many particle-removal studies based on light-scattering detection methods become unreliable.