Particle size and suspension density of urea suspensions were investigated by means of ultrasound, optical reflectance as well as focused-beam reflectance measurement techniques. These techniques provide similar proven nonrealistic results regarding suspension density and crystal size. The resulting data can be explained by the presence of bubbles. The influence of air bubbles on the measurement of different particle size and suspension density monitoring techniques is demonstrated and evaluated. Floating air bubbles as well as crystal-bubble complexes are strongly affecting the results and have to be considered under real conditions. It can be demonstrated that the negative impacts on the measured data can be decreased. Special precautions have, therefore, to be taken into account for calibrations, their validation, and during crystallization monitoring or controlling to avoid faulty measured suspension densities and particle sizes caused by bubbles.