Interaction of a Langmuir monolayer of polydimethylsiloxane (PDMS) with different hydrophobic ions added to the aqueous subphase was studied using surface pressure-area and surface potential-area isotherm techniques. For strongly hydrophobic tetraphenylborate (TPB) anions and tetraphenylphosphonium (TPP) cations, a pronounced shift of the monolayer surface potential accompanied by modest variations in the surface pressure isotherms was observed. Effects of TPB and TPP ions were found to be almost identical except the sign of a surface potential shift: it was positive for TPP and negative for TPB. An absolute value of this shift increased with concentration of hydrophobic ions and the monolayer surface density, whereas it was not affected by moderate variations of the subphase ionic strength. An effect of less hydrophobic ions (picrate anion and tetraethylammonium cation) was much less pronounced than that of TPP and TPB. A two-phase distribution model was proposed to describe the behaviour of hydrophobic ions in PDMS monolayer/subphase system. In spite of the extreme simplicity, the model appeared to describe rather well all experimental findings. It was shown that hydrophobic ions are located inside the monolayer. Their surface density is low; however, a shift of the surface potential is pronounced due to the low dielectric constant of monolayer material.