Circumstellar environment of young stars is of special interest because
 it represents the initial conditions before planet formation. Modeling
 the spectral energy distribution of young stars has led to the scenario
 of (accretion) disks around pre-main-sequence stars. However, there is a
 discrepancy in energetics. For some intermediate-mass stars the implied
 accretion rates are far too high. One possibility is excess mid-infrared
 luminosity due to non-equilibrium grains. We have recently detected the
 predicted extended emission due to non-equilibrium grains in three
 Herbig Ae/Be stars. From the ground only a few suitable cases can be
 studied despite the potentially significant luminosity contribution in
 many young stars. The problem is the low surface brightness extended
 over a large area. With the dedicated multi aperture observing mode
 (ISOPHOT AOT4) it is possible to measure the luminosity contribution of
 non-equilibrium grains to typical young stars. It is essential to
 understand the energy balance of a star and its circumstellar
 environment system before any model fitting procedure can be applied.
 ISO can provide us this crucial information.