The molecular structure of trifluorophosphine tetraborane(8), B4H8PF3, has been studied in the gas phase by electron diffraction. The experimental data can be fitted using a model which represents the gas as consisting solely of the endo conformer with C-s symmetry, the PF3 group staggered with respect to the B(I)-H(1) bond. Important experimental structural parameters (r(alpha)degrees) are r[B(1)-B(2)] (hinge-wing) = 184.7(9) pm, r[B(1)-B(3)] (hinge-hinge)= 172.2(12) pm, r[B(2)-B(3)] = 179.9(10) pm, r[B(1)-P] = 179.8(9) pm, and r(P-F) (mean) = 152.8(1) pm; B(3)B(1)P = 131.6(11)degrees, and the dihedral ("butterfly") angle between the planes B(1)B(2)B(3) and B(1)B(4)B(3) is 133.9(23)degrees. These values agree well with the ab initio (MP2/TZP level) optimized molecular geometry for the endo conformer; at the MP2/TZP//MP2/TZP + ZPE(HF/6-31G*) level, the exo conformer is predicted to represent ca. 2% of the compound vapor, consistent with the experimental B-11 NMR solution spectrum. The experimental and theoretical geometries are supported by comparison of the calculated (IGLO) B-11 NMR chemical shifts with the experimental NMR data.