An on-line double-sensor turbidimeter for estimating the solids concentration or the mean particle size in a saturated slurry system in the presence of foreign insoluble particles is proposed. The correlation representing the operation of the double-sensor turbidimeter for on-line implementation is also developed. The technique relies on measuring the transmittance of an infrared light beam through the suspension, once in the presence of soluble particles and a second time when the soluble particles have completely dissolved. The device can be easily implemented on a crystallization system for monitoring and control applications. The maximum errors for solids concentration and mean particle size measurements were 13% and 7%, respectively.