Ultrasound propagates through an elastic medium at a rate or speed named the ultrasound propagation velocity (UPV). This velocity is determined by the rate at which the wave energy is transmitted, inversely related to the square root of the density and compressibility of the medium. This technique is based on the increase of UPV with rising elasticity of the medium due to polymerization. As monomer is transformed to polymer, both the density and adiabatic compressibility change, making the UPV closely related to monomer conversion. Longitudinal waves are commonly employed in ultrasonic applications since they are easily generated and detected. 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. Another application of this technique is the direct measure of the residence time distribution (RTD), which affects both monomer conversion and polymer molecular weight distribution. RTD measurements can also be used to detect any problem of fouling or plugging during the polymerization reaction and to follow the efficiency of the reactor clean up during the shut-down procedure of continuous polymerization processes. Experimental validation of this technique during a bulk styrene polymerization is presented in this paper. Technical parameters such as temperature, pressure and flow speed, as well as medium parameters such as monomer conversion and polymer molecular weight directly influence the UPV value. The quantitative modeling of the UPV change during polymerization in order to calculate monomer conversion will be discussed in detail in a further paper (part II).