In this work, the effect of the solvent type on microwave (MW)-assisted decomposition of 2,2'-azobis(isobutyronitrile) was investigated by online monitoring of the reaction kinetics in various solvents: n-butanol, dimethyl sulfoxide, dimethylformamide, and n-propanol. The study showed that MW irradiation accelerates the reaction rates, but it does not affect the kinetic law (first order) and mechanism of the reaction. Furthermore, the Arrhenius parameters were found to be higher than their thermal counterparts, depending on the MW power density (P) as well as to the solvent type. On the other hand, two new properties were proposed to investigate the solvent/MW effects: a solvent property (S) defined by means of Hansen solubility parameters and the relaxation time of the solvent (tau). According to experimental results, solvents with high S and with an optimum tau value are favorable for an enhanced MW effect on chemical reaction kinetics, pointing to specific molecular structures for solvent candidates.