Ultrasound propagates through an elastic medium at a rate or speed named the ultrasound propagation velocity (UPV). This velocity is detd. by the rate at which the wave energy is transmitted, inversely related to the square root of the d. and compressibility of the medium. This technique is based on the increase of UPV with rising elasticity of the medium due to polymn. As monomer is transformed to polymer, both the d. 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 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. Another application of this technique is the direct measure of the residence time distribution (RTD), which affects both monomer conversion and polymer mol. wt. distribution. RTD measurements can also be used to detect any problem of fouling or plugging during the polymn. reaction and to follow the efficiency of the reactor clean up during the shut-down procedure of continuous polymn. processes. Exptl. validation of this technique during a bulk styrene polymn. is presented in this paper. Tech. parameters such as temp., pressure and flow speed, as well as medium parameters such as monomer conversion and polymer mol. wt. directly influence the UPV value. The quant. modeling of the UPV change during polymn. in order to calc. monomer conversion will be discussed in detail in a further paper (part II). [on SciFinder (R)]