SCIENTIFIC ABSTRACT Although there is now compelling evidence that many young stellar objects (YSOs) are surrounded by disks, e.g. the classical T Tauri stars (CTTSs) and some Herbig Ae/Be stars, it is not clear how, and at what rate, such disks evolve. Certainly by the time these stars have reached the main sequence, most of the disk material has either disappeared or coagulated into larger bodies. What causes a disk to dissipate: is it accretion, the presence of winds, or the formation of larger bodies like planets? In order to address this problem one must look at stars older than the CTTSs since these stars are at most a few Myrs old and it takes typically a 100 Myrs before they finally settle on the ZAMS. The region in the HR diagram between the CTTSs and the main sequence is occupied by the post-T Tauri stars (PTTSs). It is proposed to obtain multi-band photometry with PHT of a sample of evolved pre-main sequence stars to measure their mid- to far-infrared spectral energy distributions. By comparing the spectral energy distributions of these stars with those of CTTSs it is hoped to see evidence for the evolution of circumstellar disks from the accretion phase right through to the dissipation of the surrounding dust and perhaps the earliest stages of planetary formation. Another fundamental question it is intended to address, in connection with disk evolution, is whether weak-line T Tauri stars (WTTSs) are simply CTTSs devoid of circumstellar matter, as has been claimed. WTTSs and CTTSs are located in the same region of the HR diagram and are of approximately the same age. The increased sensitivity of ISO, in comparison with IRAS, will allow for a much deeper search for material around WTTSs than has hitherto been possible. OBSERVATIONAL SUMMARY The target list is largely made up of sources that were not detected by IRAS or their fluxes are poorly known. For those objects which are already known IRAS sources, observations will be made with 9 filters: P_4.85, P_7.3, P_10, P_11.5, P_12.8, P_16, P_20, P_60 and P_100 using AOT PHT03 to obtain much more detailed spectral energy distribution curves than has hitherto been possible and to test for silicate and PAH features. Here the on-source integration time will be 16 sec for all filters with an equal amount of time spent off source in staring mode. For the P_4.85, P_7.7, P_10, P_11.5 and P_12.8 observations an aperture of 10 arcseconds will be used For the P_16 and P_20 observations an aperture of 23 arcseconds will be employed and finally an aperture of 79 arcseconds for the P_60 and P_100 observations. In the case of IRAS non-detections (or occasionally for very faint IRAS sources) we will observe only with the four broadband filters P_11.5, P_25, P_60, P_100 and the silicate filter P_11.3. Here the on-source integration time will be 32 sec for all filters with an equal amount of time spent off source in staring mode. For the P_11.3, P_11.5 and P_25 observations an aperture of 23 arcseconds will be used. For the P_60 and P_100 observations an aperture of 79 arcseconds will be employed. Those stars of the this group that show evidence for disks will be observed in a follow-up programme at additional wavelengths.