During most of their life galaxies are quiescent. Sometimes, however, they experience short and violent phases of star formation, usually in their nucleus. To study the transition from quiescence to activity, we propose to obtain detailed spectral FIR energy distributions for two complete samples of active and inactive spirals. This will allow us to determine the total gas mass which serves as the fuel for the star bursts. Our ground based observations from 450 to 1300 microns together with IRAS data show that a precise knowledge of the spectral shape around 100 microns is needed for a unique decomposition of the dust emission spectra and for a reliable determination of the dust and gas mass. The efficiency of star formation, expressed by the ratio of IR luminosity over gas mass, will be determined for these samples. We will probe the radiation field in the nucleus of the galaxies with CAM. The PAH bands will be used as a descriminator of the activity type of the galaxies. We will use state-of-the-art radiative transfer models to explain the spectral energy distribution of the galaxies and compare the observations with predictions from our dynamical models.