Systematic studies on the LET effect in the radiolysis of organic liquids have been initiated using various types of heavy ions accelerated by cyclotron. First, the effects of differences in irradiation conditions, in particular beam current, stirring speed and concentration of solutes have been examined. Aerated aqueous ferrous sulfate solutions (Fricke solutions) were subjected to irradiation by protons and He ions of various energies, to take advantage of the well-established reaction mechanism of the system. Experimental results obtained under various conditions were analyzed on the basis of the diffusion kinetics model. It was revealed that meticulous care must be taken to control the experimental conditions in studying radiolysis with heavy ions. The oxidation yields of ferrous ions were also determined as a function of linear energy transfer.