A continuing effort is being made in the development of on-line sensors for the monitoring and control of crystallization processes. Applied on-line calorimetry provides an estimate of the instantaneous heat release due to crystallization advancement. Two approaches to the implementation of such "soft sensors" are described and difficulties arising in the case of on-line multipurpose non-isothermal applications art outlined. Off-line estimates of concentration and supersaturation profiles during typical batch evaporative crystallizations are reported. Particular attention is paid to the detection of second, late nucleation bursts occurring in cases of poor crystal growth and rapid temperature decay.