Ultraviolet irradiation of formamide in solid argon forms hydrogen-bonded, carbon- and oxygen-attached complexes NH3-CO and NH3-OC. Computationally, the carbon-attached complex is 1.2 kJ mol(-1) more stable than the oxygen-attached complex. However, a thermal equilibrium of the two structures is found experimentally in the matrices. Moreover, the oxygen-attached complex dominates in the 193-nm-induced photolytic complex formation from formamide. In xenon matrices, the photochemistry of formamide increasingly yield the HNCO+H-2 binary system. The change in photochemistry of formamide between the argon and xenon enviroments can be attributed to an external heavy-atom effect, where xenon enhances the rate of intersystem crossing from a singlet to a triplet surface.