In this work we show that it is possible to use MALDI-TOF as a tool to quantify the atomic composition and to describe the phase segragation of the surface of ligand-coated, bimetallic AuAg nanoparticles. Our investigation shows that AuAg nanoparticles of various compositions exhibit core-shell heterogeneity with surface enrichment of Ag. A Monte-Carlo type simulation demonstrates that the surface Au and Ag atoms arrange in a random fashion.