Sketch of a section of a disk. On the left hand side the region in red shows where CO isotopologues are expected to survive, if isotope-selective dissociation is not included. On the right hand side isotope-selective effects are included and the emitting region in red is reduced. Panel a) shows the case of a warm disk, where CO isotopologues do not freeze-out anywhere in the disk, while panel b) shows the case of a colder disk where freeze-out occurs in the midplane. The combination of freeze-out and isotope-selective photodissociation (panel b), right hand side) drastically reduces CO isotopologues emission.