The formation of ultrasmall PbS nanoparticles by the reaction of lead oleate with bis(trimethylsilyl) sulfide in octadecene at a temperature as low as 50 °C can be strongly promoted by addition of protic solvents (alcohols, water) or acetone. The proposed mechanism involves formation of the Pb(oleate)2-(TMS)2S complex which upon the injection of a proper solvent forms highly reactive Pb- and S-containing intermediates. The different reactive ability of these intermediates depending on the type of promoter allows one to prepare PbS QDs with different size. Such a mild-temperature process is useful for production of ultrasmall monodisperse PbS colloidal quantum dots due to suppressed recrystallization of PbS nanoparticles.