PROBLEM TO BE SOLVED: To provide an automatic dilution system and method that provides optimal dilution factor (DF) for a sample suspension containing particles mixed with a diluent. SOLUTION: A sensor measures the value of a particular characteristic, such as, the initial rate of increase of a quantity related to particle concentration. The optimal value of DF is calculated, and a control signal is developed, to adjust the flow of the sample to provide the optimal value of DF. In one embodiment, only a portion of the sample is directed through the sensor for shortening the time to reach equilibrium. The time to reach equilibrium is further reduced in another embodiment in which the mixing means is a static mixer, having relatively small volume. Larger dilution factors are obtainable in a further embodiment, in which the sample is prediluted in a mixing chamber and then further diluted in a second mixing chamber. COPYRIGHT: (C)2011,JPO&INPIT