The geodetic IRIS-network is a transatlantic 5-station VLBI-array dedicated to monitor variations of the earth's rotation (e.g. Carter et al., 1985). In a 5 day cycle 14 extragalactic radio sources are observed in regular intervals, covering a GST-range of up to 24 hours, using the MkIII-system in Mode C at 3.6cm (X-band) and 13cm (S-band) wavelength. As this database provides a great potential to monitor source structures and their changes with mas-resolution (the sample contains at least 7 “superluminals”), we have initiated a project to obtain radio maps at both frequencies and to determine, how accurate physical parameters like component flux densities and separations can be measured. Early results are published in Schalinski (1985) and Schalinski et al. (1986). In addition to astrophysical applications, we investigate the influence of structure phases on phase- and group-delay corrections (s. Campbell et al., this vol.).