Reactions of laser-ablated U atoms with P 4 molecules upon codeposition in excess argon gave weak new infrared absorptions at 504, 483, and 426 cm -1 , which are best identified as binary uranium phosphide UP4 species based on extensive B3LYP, BPW91, and PBE density functional and CASSCF/CASPT2 wave function based calculations. These UP 4 adducts may be considered as simple models for P 4 activation products by ligand-supported transition and main group metal complexes. The sought-after PUP molecule in the 3 Φ u ground state probably absorbs under the intense P 4 precursor band near 465 cm -1 . The triplet U(IV) molecule PUP is 7 kcal/mol lower in energy than the U(VI) analog, singlet PUP. The 3 Φ u ground state of PUP, with 2.54 effective bond order computed by CASPT2, may be compared to the 1 Σ g + ground state of N=U=N with 2.87 effective bond order, where third row elements are less effective for πbond formation than second row elements.