Inline conversion measurements in chemical reactions are in general and especially in polymerization reactions necessary for process monitoring and control. For polymerization 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 density 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 polymerization. The application of the UPV technique to follow the composition evolution of the solution during the polymerization 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 laboratory and industrial purposes.