The effects of inorganic salts (Na_2SO_4 or NaCl) on the rates of sonochemical degradation of phenolic compounds (4-chlorophenol, phenol, catechol and resorcinol) were investigated under argon. It was suggested that the rate of sonochemical degradation of phenolic compounds was affected by the following two points: the changes in argon solubility in the solution and the changes in pseudo hydrophobicity (diffusion behavior) of phenolic compounds. The decrease in argon solubility at higher salt concentration resulted in the low sonochemical efficiency: the yield of H_2O_2 formed decreased with increasing salt concentration. This effect should decrease the rate of degradation of phenolic compounds. In addition to this effect, it was suggested that phenolic compounds more easily accumulated at the interface region of the bubbles at higher salt concentration, resulting in the increment of the rates of sonochemical degradation. This effect should increase the rate of degradation of phenolic compounds. Taking into account these effects, the observed salt effects would consist of a positive and negative effect so that the salt effects were observed as an unclear effect.