The electron-diffraction pattern from gaseous P4O6 has been recorded over the range s= 1.24 to 50.00 Å–1. Structure analysis gave results which show no deviation from the accepted tetrahedral cage structure. The final molecular parameters are: P—O = 1.638 ± 0.003 Å, ∠P—O—P = 126.4 ± 0.7°. The comparable parameters in P4O8, as determined by least-squares refinement using Jost's new crystallographic data are P—O = 1.633 ± 0.010 Å, ∠P—O—P = 130.1 ± 0.9°. A survey is made of the dimensions of the four oxides P4O6, P4O8, P4O9, P4O10. Four different P—O bonds are distinguished and their mean lengths determined: P—O (peripheral)= 1.424 ± 0.004 Å; Pv—O (bridge)= 1.595 ± 0.004 Å; PIII—O(PIII)= 1.637 ± 0.004 Å; PIII—O(Pv)= 1.666 ± 0.008 Å. The dimensions of P4O7 are predicted.