Observations of submillimeter lines of CO, HCO+, HCN and their isotopes from circumstellar disks around low-mass pre-main sequence stars can be used to set constraints on the temperature and density distributions in these disks. The lines considered here originate from levels with higher excitation temperatures and critical densities than studied before (CO 6–5, HCO+ and HCN 4–3), and are combined with interferometer data on lower excitation lines. We discuss the results for two disks, i.e., those around LkCa 15 and TW Hya. We find that the TW Hya disk has a warm surface layer and agrees well with a flaring disk geometry, while the LkCa 15 disk is cooler and can be described by either dust-settling in a flared disk or a flatter disk overall. The densities are well described by disk models in the literature.