Recent systematic studies of the basic properties of type SRa and SRb
 Semiregular variables (SRVs) have found that the SRbs consist of a
 "blue" group with no circumstellar shells, short periods and higher
 temperatures and a "red" group with temperatures and mass loss rates
 comparable to Miras but periods about a factor of two smaller. Near IR
 photometry shows that the "blue" SRVs are similar to non-variable
 giants, while the "red" SRVs are intermediate objects between Miras
 and normal giants. The difference in atmospheric structure, most
 prominently seen in the strength of the water bands, could be due to
 the smaller pulsation amplitudes and weaker shockfronts of the SRVs.
 Thus, when compared to Miras and non-variable giants, the "red" SRVs
 can provide important clues for the influence of the pulsational
 behaviour on the atmospheric structure and mass loss of AGB stars.
 Many of the theoretical tools for interpreting the spectra of
 these stars are available now. Since no detailed spectroscopic studies
 in the IR exist, we propose to obtain SWS spectra of selected "red"
 SRVs. These will be compared with state of the art theoretical models.
 The SWS on board of ISO covers most of the important molecular bands of
 H2O, CO and SiO and the wavelength region where continuous dust
 emission starts to dominate. While some of the mentioned bands can be
 observed from the ground, this requires different instruments and often
 excellent observing conditions. The use of different instruments
 generally means that different molecular bands and the dust emission
 cannot be studied at the same time, a severe disadvantage in view of
 the variability of AGB stars and the pronounced coupling  between dust
 and different molecules. Only ISO can provide, probably for a long
 time, complete, simultaneous and homogeneous data of the molecular
 bands and the dust emission.