We study the effect of self-assembled monolayer (SAM) treatment of the SiO2 dielectric on the electrical characteristics of PbS transistors. Using SAMs, we observe threshold voltage shifts in the electron transport, allowing us to tune the electrical properties of the devices depending on the SAM molecule used. Moreover, the use of a specific SAM improves the charge carrier mobility in the devices by a factor of three, which is attributed to the reduced interface traps due to passivated silanol on the SiO2 surface. These reduced traps confirm that the voltage shifts are not caused by the trap states induced by the SAMs.