The dielectric properties of the bulk and at two grain boundaries of commercial SrTiO3 polycrystals have been determined by analysing the low loss region of the electron energy loss spectrum in a transmission electron microscope. For this purpose, we have made use of field emission scanning transmission electron microscopes equipped with electron energy loss spectroscopy systems which provide subnanometre electron probes and offer energy resolutions of 0.3 eV and 0.7 eV, respectively. In this paper we calculate the dielectric function by a Kramers-Kronig transformation and also the interband transition strength. The interband transitions arise from maxima in the valence-band density of states. These transition calculations compare well with published optical data and theoretical calculations. The JDOS of the boundaries is obviously and reproducibly different from that in the bulk.