The HDDR (hydrogenation, disproportionation, desorption, recombination)-process of bulk Nd-Fe-B-type alloys has been investigated by in-situ electrical resistivity measurements. The effect of hydrogen pressure on the rate of the disproportionation reaction, in which the Nd2Fe14B phase forms Fe, Nd-hydride and Fe2B, was studied in detail on a Nd14.68Dy0.94Al0.62Fe76.47Nb0.5B6.79 alloy, which is used for the production of commercial permanent magnets. Experimental data reveal a strong dependence of the HDDR-kinetics on the hydrogen pressure which was varied between p(H-2)=0.1-1.0 bar at T=750-degrees-C. Optical micrographs suggest that the Nd-rich grain boundary phase acts as a diffusion path for the hydrogen in the HDDR-process. The 2-14-1 matrix phase subsequently disproportionates from the grain boundary phase inwards.