  We discuss a technique for mapping the synchrotron turnover frequency
distribution using nearly simultaneous, multi--frequency VLBI observations. The
limitations of the technique arising from limited spatial sampling and
frequency coverage are investigated. The errors caused by uneven spatial
sampling of typical multi--frequency VLBA datasets are estimated through
numerical simulations, and are shown to be of the order of 10%, for pixels with
the deconvolution SNR~7. The fitted spectral parameters are corrected for the
errors due to limited frequency coverage of VLBI data. First results from
mapping the turnover frequency distribution in 3C345 are presented.
