The electron-diffraction pattern from gaseous P4O10 has been recorded on the Oslo apparatus and intensity data obtained over the range s= 1.16 to 45.50 Å–1. The data were processed and the molecular structure was refined on the KDF9 computer in Glasgow, using a comprehensive new series of programmes. The results confirm the tetrahedral cage structure of the molecule. The refinement, which was by least-squares, produced the following molecular parameters: P—O(per)= 1.429 Å; P—O(br)= 1.604 Å; ∠P—O(br)—P = 123.5°. The reproducibilities are 0.004 Å, 0.003 Å and 0.7°, respectively. These results are in agreement with the dimensions of the P4O9 molecule as determined by a least-squares refinement using Jost's crystallographic data.