HNC/HCN column density ratio calculated with the PDR model for a diffuse cloud as a function of the density of H nuclei. Dashed lines correspond to models where the photodissociation rate of HNC is assumed to be equal to that of HCN, while solid lines correspond to models where HNC is photodissociated 2.2 times faster than HCN. The range of HNC/HCN ratios observed by Liszt & Lucas (2001), 0.21 ± 0.05, is also shown.