The results of accurate SCF calculations on P2 and PO are reported. At the equilibrium distance Re, the d population is about 0.34 electron for the two P atoms in P2, 0.29 electron for the P atom in PO, and 0.05 electron for the O atom; this d participation approximately confirms the results of Boyd and Lipscomb. As R decreases, d participation in P2 increases markedly. At Re, deletion of d and f functions from the basis set increases the computed energy by 0.093 au (2.53 eV) in P2 and 0.114 au (3.10 eV) in PO. Comparisons with Cl2 and N2 are discussed; d and f participation is smaller, but in N2 its energetic effect is about the same (0.095 au or 2.58 eV) as in P2; in Cl2 the energy effect is somewhat smaller (0.059 au or 1.61 eV). It is argued that d participation in second-row atoms, and even in first-row atoms, when they enter molecules, can properly be thought of as a chemical effect. The contribution of free isovalent s-pa hybridization to bonding in N2 and P2 is explored by deleting valence-shell p functions from the LCAO functions used by the valence-shell σu MO in these molecules. By these deletions the energy is increased by 0.12 au (3.3 eV) in N2 but only by 0.02 au (0.5 eV) in P2; this difference helps to explain the difference in properties of N2 and P2. Some additional probable free hybridization in the σg MO's is also discussed. In F2 the energy effect of d participation is about half as large as in Cl2. © 1971, American Chemical Society. All rights reserved.