It was found that a P-doping process occurs in Nd2Fe17 by gas phase reaction around 673 K. A decrease of the lattice constants a and c by up to 0.6 and 0.8%, respectively, and an increase of the Curie temperature by up to 30 K were observed. The P-doped 2:17 phase dissolves around 763 K. Four phase regions in the phase diagram of Fe rich NdFeP alloys below 1223 K were determined. The phases in each region are: A: Nd2Fe17 + α-Fe + NdP; B: α-Fe + NdP + Nd2Fe12P7; C: α-Nd2Fe12P7 + Fe3P; D: Nd2Fe12P7 + Fe2P + Fe3P. The variation of the lattice constants with composition of the alloys is as large as (Δa/a)max 0.8% for NdP and (Δa/a)max 1.2% and (Δc/c)max 0.8% for Nd2Fe12P7 suggesting a certain soluble range for the compounds.