A flowing argon stream containing N-2 was Seeded with P-4 vapor, subjected to microwave discharge, and condensed at 12 K. Weak infrared absorptions due to PN (1327.7 cm((-)1)), P2O (1270.7 cm(-1)), P2O (1240.8 cm(-1)), SiO (1226.0 cm(-1)), P-4 (465 cm(-1)), and PO (1218 cm(-1)) and a strong new 1179.3 cm(-1) band were observed. The new absorption is assigned to the linear PNP radical on the basis of N-15 substitution, annealing and photolysis behavior, and complete-active-space self-consistent field (CASSCF) structure and frequency calculations. A 353 nm band system with a 560 +/- 20 cm(-)1 vibronic interval is assigned to the A(2) Sigma(u)(+)-X(2)IIg transition of PNP with the help of large-scale multireference calculations.