  We present the results of phase-referenced VLBA+Effelsberg observations at
five frequencies of the double-image gravitational lens JVAS B0218+357, made to
establish the precise registration of the A and B lensed image positions. The
motivation behind these observations is to investigate the anomalous variation
of the image flux density ratio (A/B) with frequency - this ratio changes by
almost a factor of two over a frequency range from 1.65 GHz to 15.35 GHz. We
investigate whether frequency dependent image positions, combined with a
magnification gradient across the image field, could give rise to the anomaly.
Our observations confirm the variation of image flux ratio with frequency. The
results from our phase-reference astrometry, taken together with the lens mass
model of Wucknitz et al. (2004), show that shifts of the image peaks and
centroids are too small to account for the observed frequency-dependent ratio.
