The implantation of 150 keV Sb+ ions to doses from 5 X 10(12) to 1 X 10(15) cm-2 into polyethyleneterephthalate (PET) and polybutyleneterephthalate (PBT) was studied. The sheet resistivity of the ion beam modified polymers was found to be a decreasing function of the implanted dose with more significant decrease being observed on the PET samples. The latter observation is in correspondence with UV-VIS measurements proving higher production of conjugated double bonds in PET comparing to PBT. The dissimilar behaviour of PET and PBT is explained by a different number of methylene groups in polymer chains. Approximately the same thickness of the layer modified by ion implantation was found in Rutherford back-scattering (RBS) measurements for both polymers. A variable range hopping mechanism is tentatively suggested to be responsible for the electrical charge transport.