Abstract - The Earth-Moon system has unique chemical and isotopic signatures compared to other planetary bodies; any successful model for the origin of the Earth-Moon system has to satisfy these chemical and isotopic constraints. The Moon is substantially depleted in volatile elements such as potassium compared to the Earth and the bulk solar composition, and it has long been thought to be the result of a catastrophic Moon-forming Giant Impact. Volatile element depleted bodies like the Moon were expected to be enriched in heavy potassium isotopes because of the loss of volatiles to space, but such enrichment was never found. Here we report new high-precision potassium isotopic data for the Earth, Moon and chondritic meteorites. We found that the lunar rocks are significantly enriched in the heavy isotopes of potassium compared to the Earth and chondrites. The enrichment of heavy isotopes of K in lunar rocks compared to those of Earth and chondrites can be best explained as the result of the incomplete condensation of a Bulk Silicate Earth (BSE) vapor at an ambient pressure higher than 10 bars. We compared these coupled constraints of the chemical loss and isotopic fractionation of K to two recent dynamic models that were used to explain the identical non-mass dependent isotopic composition of the Earth and Moon. Our K isotope result is inconsistent with the “low energy disk equilibration” model, but consistent with the “high energy, high angular momentum giant impact” model for the origin of the Moon.