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 UP(4) 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 π bond formation than second row elements.