The asteroid belt contains a record of the thermal evolution of the early solar-system, with the more distant asteroids containing volatile compounds that escaped heating by the sun. The reddish, low-albedo D-type asteroids that dominate the outer asteroid belt possess optical properties characteristic of organic compounds. Unfortunately their exact surface composition remains unknown, as diagnostic bands of organic molecules lie at short infrared wavelengths (2-12 microns), and few are accessible from ground-based observatories. We propose to investigate this wavelength region in 4-5 D-type asteroids with ISO. Our primary aim is to identify the exact surface composition of these bodies. By comparing individual asteroids, degrees of variation in composition will be searched for. Findings from this investigation will provide the first observations by which previous laboratory studies may be compared.