Filter efficiency determination by means of the optical particle counting technique can yield information about the filter performance from a single set of upstream and downstream measurements. In principle, optical particle counters provide the possibility of a real time determination of particle size distribution and concentration. However, substantial errors in determination of the filtration efficiency may result from the performance characteristics of optical particle counters. This article presents results of a theoretical and experimental study analyzing the effects of variation of particle refractive index and coincidence on the sizing accuracy and counting ability of optical particle counters and the impact on the efficiency determination. Since both parameters, size and count, are critical for the determination of fractional collection efficiency and the most penetrating particle size, exemplary filter efficiency measurements with optical particle counters under various conditions are presented and critically discussed. Depending on the experimental conditions chosen, the efficiency of the same filter can differ by more than an order of magnitude. The limitations reported here may numerically vary for different instruments; however, they are inherent to this technique, in gaseous as well as in liquid media, and must be taken into account during measurements and data evaluation.