Reactions of laser-ablated U atoms with P4 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 UP4 adducts may be considered as simple models for P4 activation products by ligand-supported transition and main group metal complexes. The sought-after PUP molecule in the 3Φu ground state probably absorbs under the intense P4 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 3Φu ground state of PUP, with 2.54 effective bond order computed by CASPT2, may be compared to the 1Σg+ 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. © 2010 American Chemical Society.