This paper is the second part of a description of the online monitoring through ultrasound propagation velocity measurements of styrene polymn. in a continuous recycle reactor. A set of calibrations and a simple math. model are proposed to interpret the exptl. ultrasound propagation velocity measurements during the course of bulk and soln. polymns. 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 exptl. results. [on SciFinder (R)]