Disk-integrated ¹³CO (top), C¹⁸O (middle), and C¹⁷O (bottom) line intensities (J = 3−2) obtained with disk masses M_d = 10^-4,10^-3,10^-2M_⊙ are shown as a function of the disk mass. The beam size is 3′′. The red line relates the intensity values obtained for the three disk masses with the NOISO network, while the blue line shows those with the ISO network. Dotted lines indicate by which factor the total disk mass is underestimated if no isotope-selective processes are considered. Results are presented for f_large = 10^-2, 0.99 and for T Tauri and Herbig disks.