Pressure tuning of spectral holes provides rich information for the host-guest system under investigation: It is possible to measure the compressibility of the host, the vacuum absorption frequency of the guest, and the absolute magnitude of the solvent shift. Further, details of the guest-host interaction and the elastic-versus-plastic deformation behavior can be inferred. It is possible to determine the volume fluctuations of chromoproteins, an important quality with respect to their functioning.