Although 16O18O is a minor constituent of the atmosphere, a recent estimate indicated that its photodissociation rate may be significant because of selective absorption in the region of the Schumann-Runge bands as a result of isotope effects in the spectrum. We have calculated the 16O18O dissociation rate coefficient (J34) using a line-by-line model of the cross section in the band system and have found that at altitudes between 50 km and 80 km it is of the order of 10 times the dissociation rate coefficient for 16O2 (J32). The total 16O18O dissociation rate contributes a maximum of 3% of the O2 photodissociation, an order of magnitude less than the previous estimate. Theoretical values of the spectroscopic constants for 16O18O are given.