Crystallization of poly(ethylene terephthalate) (PET) is accompanied by significant changes in surface topography, easily detected by atomic force microscopy (AFM). Phase imaging by AFM qualitatively indicates contrast in mechanical properties of nanometer scale areas of an annealed PET surface, but cannot provide quantitative data. Using interfacial force microscopy (IFM), we have, for the first time, made quantitative measurements of the elastic moduli of such nm-scale areas on a homopolymer surface. Values of 2.2 GPa, 4.3 GPa, and 11.8 GPa, were found, respectively, for amorphous PET and for phase segregated regions on the surface of an annealed homopolymer PET sample. The method is applicable to any phase segregated surface with nm-sized domains of differing elastic moduli.