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.