High resolution electron energy loss spectroscopy (HREELS) is used to study spin-coated films of two polyaminoacids, poly(beta-benzyl-l-aspartate) (PEA) and poly(gamma-benzyl-L-glutamate) (PEG). The experimental results show that the side chains of these two polymers take a very different conformation at the surface : This is confirmed by infrared spectroscopy (ir) and differential scanning calorimetry (DSC) which are more bulk sensitive techniques. This effect is further attributed by computer graphics facilities (BIOSYM) to a larger flexibility of the side chain of PEG as compared to PEA.