The extraction of active principles or oils from vegetable products by means of an appropriate solvent is a classical unit operation in chemical engineering. The aim of the present work is to improve the yield or the selectivity of the extraction using ultrasound. Two solid-liquid extractions have been investigated: pyrethrines from pyrethrum flowers (Chrysanthemum cineraria) and oil from woad seeds (Isatis tinetoria). In the case of the first one, an acceleration of the kinetics and of the yield of the extraction has been obtained, probably linked to the increase of the intraparticular diffusion of the solute, which is the limiting step of mass transfer. On the other hand, ultrasound seems to have a weak effect in the case of woad seeds, probably because of the structure of the seeds. In order to distinguish the various effects acting on the ultrasound activation, a thermoelectric probe has been built for measuring the ultrasonic intensity profile. By this method, it is possible to investigate the influence of the presence of solid particles on the ultrasound wave propagation.