Palladium-tin nanocolloids have been analyzed with high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS). The composition of individual colloids with a diameter of 2-5 nm has been deduced. It has been established that the colloids consist of a core of a PdxSn1-x alloy, with x ranging from 0.6 to 1. From the numerical comparison of experimental EELS line scans with reconstructed ones from a model colloid, it has been possible to evidence a slight Sn surface enrichment equivalent to a submonolayer of pure Sn on the surface of the colloid.