A direct comparison between the primary air kerma standards of the PTB and OMH was carried out for 60Co and 137Cs y rays in June 2000 within a time period of 1 month. The OMH standard is a graphite cavity chamber of the cylindrical type. The PTB standard is comprised of three graphite cavity chambers, different in shape and size. Two of them are of the cylindrical type and one of them is a pancake type chamber. The main technical parameters of the standards and the applied beams are described. The correction factors of the chambers are given together with their uncertainties. In particular, it is pointed out that for the first time calculated instead of measured wall correction factors were applied to all chambers.weiterlesen