Inline conversion measurements in chem. reactions are in general and esp. in polymn. reactions necessary for process monitoring and control. For polymn. monitoring, the sound propagation (UPV) can be related to the texture of the bulk with a high accuracy. As the monomer is transformed to polymer, both the d. and adiabatic compressibility change, making the UPV closely related to the monomer conversion. This technique is based on the increase of UPV with rising elasticity of the medium due to the polymn. The application of the UPV technique to follow the compn. evolution of the soln. during the polymn. reaction in a recycle tubular reactor is presented. The sensor can be directly inserted in the tubular reactor and does not require a sampling circuit. This makes its use particularly simple and suitable for lab. and industrial purposes,.