A method was developed for the quantification of cow milk adulteration in goat milk, based on solvent separation of whey proteins followed by high-performance liquid chromatography with electrospray ionization mass spectrometry (HPLC/ESI-MS). The presence of cow milk was determined using beta-lactoglobulin whey protein as the molecular marker. The adulterants were identified using both retention time and molecular mass derived from multiply charged molecular ions. Standard solutions containing cow and goat milk in different volume ratios were prepared and analyzed. Good linearity covering cow milk content from 5% and above was obtained. The proposed method identifies the adulterants using accurate molecular masses for protein identification and detects the addition of cow milk to goat milk at levels as low as 5%.