This paper is the second part of a description of the on-line monitoring through ultrasound propagation velocity measurements of styrene polymerization in a continuous recycle reactor. A set of calibrations and a simple mathematical model are proposed to interpret the experimental ultrasound propagation velocity measurements during the course of bulk and solution polymerizations. The validity of this new method is shown by comparison of values obtained by Ultrasound Propagation Velocity (UPV) and traditional techniques. The model allows a good description of the experimental results.