We plan to use the reverberation technique to map the physical
 conditions and structure of the molecular torus in radio quiet Seyfert
 I galaxies. Specifically, we plan to monitor the 2.5-12 micron flux of
 one galaxy every 15 days during 14 months with PHT-S. Contemporaneous
 optical and UV spectra will be secured. By cross-correlating the IR
 and optical-UV light curves, it will be possible to measure the delay
 in the response of the dust and molecular band emission to the
 variations of the UV-optical flux. In close to face-on objects such
 as Seyfert I Active Galactic Nuclei (AGN), this delay is a measure of
 the light-travel time to the dust reprocessing region and thereby of
 the torus inner radius. By measuring the delay and variability
 amplitude as a function of wavelength, it will be possible to infer
 the dust temperature gradient and constrain the opacity gradient
 inside the torus. Delayed variations of the H2 and CO lines will yield
 information on the structure and physical conditions of the gas inside
 the torus. Delayed IR variations have been observed in several AGNs.
 In 3 cases, it has been possible to show that the torus inner radius
 corresponds closely to the dust sublimation radius and is about 2-3
 times as large as the broad emission line region (BLR). Given the
 luminosity of our targets and the radius of their BLR, one can
 therefore set a conservative upper limit of 100 days to the delay of
 the 3.5 micron flux. Furthermore, for one target (NGC 1566), the delay
 has been directly measured to be about 50 days, well within reach of
 our planned observing campaign.