The crystal structures of the compounds tris(para-chlorophenyl)phosphine oxide {(p-ClPh)3PO} and tris(para-methoxyphenyl)phosphine oxide {(p-OMePh)3PO} were determined by X-ray diffraction methods. (p-ClPh)3PO crystallizes in the space group P-1 (no. 2) with a = 11.828(2), b = 12.645(2), c = 14.072(2) Å, α = 97.90(1), β = 109.45(1), γ = 115.43(1), V = 1692.3(2) Å3 and Z = 4. The mean O–P and C–P distances are 1.481(6) and 1.806(2) Å, respectively, and the mean C–P–C angle is 106.5(1.1). (p-OMePh)3PO crystallizes in the space group P21/c (no. 14) with a = 18.8642 (10), b = 10.3999(5), c = 21.3462(16) Å, β = 115.414(6)°, V= 3782.6(4) Å3, and Z = 8. The mean O–P and C–P distances are 1.484(5) and 1.798(4) Å, respectively, and the mean C–P–C angle is 106.5(1.0). These two structures were analyzed along with the previously determined structures of triphenylphosphine oxide {Ph3PO} and tri-p-tolylphosphine oxide {(p-MePh)3PO}, and IR data were collected on all four compounds. Both the observed P–O distances and the IR stretching frequencies for these triarylphosphine oxides support the interpretation of the P–O bond as having substantial multiple-bond character, with a bond order between 1.7 and 1.8. The para-substituents on these triarylphosphines were shown to have a statistically insignificant effect on the P–O bond.