Laboratory experiments were used to determine the thermal (Soret) isotopic fractionation of lithium and potassium in a basalt melt, which adds elements with ionic charge +1 to the list of elements for which thermal isotopic fractionations in silicate liquids have been previously reported (i.e., Ca, Mg, Fe, Si, O, Sr, Hf, and U). The new experiments were run at a moderate pressure of about 1.5 GPa in a piston cylinder apparatus in order to avoid gas bubbles once the sample was melted. The samples were displaced slightly below the hot spot of the piston cylinder assembly graphite furnace so that there would be a temperature difference of about 125° C across the samples while molten. The thermal isotopic fractionation factor Ω (per mil fractionation per 100° C per one atomic mass unit difference) was found to be 6.0 for lithium isotopes and 1.1 for potassium isotopes. The isotopic fractionation in both cases resulted in the heavy isotopes becoming enriched at the cold end. The expanded data set of thermal isotopic fractionation in silicate liquids is used to evaluate the degree to which recently proposed parameterizations are able to reproduce the experimental data.