Abstract: An on-line slurry ash monitoring system for evaluating the composition of coal slurries in coal preparation plants. A coal slurry is introduced into a sample reservoir where it is removed and passed through a closed loop slurry transport system which causes the sample to be directed through a sensor and recirculated back to the reservoir. In order to enhance sensor accuracy, a sample is exposed to vacuum pressure conditions which removes gas bubbles and dissolved gases from the sample. The sensor is preferably a radiation absorption type in which characteristic absorptions of coal particles, ash and iron are evaluated by associated electronics to provide an output of slurry composition.