Anomalous x-ray synchrotron diffraction was used to determine the crystallographic parameters of PrFeCoBbased magnetic alloys. The effect of cobalt concentration on the crystallographic parameters of the magnetically hard Pr2Fe14???xCoxB phase was studied. The results indicate that addition of cobalt has a marked effect on crystal structure. Variation of the c parameter decreased twice as much as the a parameter with increase in Co content. The positions of inequivalent atoms of the magnetically hard matrix phase ?? in the Pr-based alloys were determined using Rietveld refinement. This permitted determination of the relative distance of each inequivalent atom from its nearest neighbors. Cobalt occupied the 16k2 site and Fe had a tendency to occupy the 8j2 sites located between the Kagom?? layers.