We propose to use ISO to carry out high-sensitivity spectroscopy of molecular bands around massive young stellar objects (YSOs). Infall and outflow are expected to take place simultaneously around these objects, and we believe that ISO equipped with the SWS and the LWS will be the best instrument in many years to study such processes by the means of IR spectroscopy. The outflows will be studied by the observation of the purely rotational lines of key molecules such as CO, OH, CH, CH+, and NH3 in the spectral window of the LWS. We plan to observe several transitions to carry out a multiline analysis to obtain the physical parameters of the gas. The infalling gas will be studied by observing prominent bending bands of abundant molecules such as HCO+, HCN, HNC, HC3N, NH3, CCH, CO2, CS, N2H+, CH3OH, in absorption against the strong central IR sources by using the SWS. The absorption features are expected to be formed in the circumstellar region, and will provide crucial information about the infall processes taking place at the very vicinity of the young star. This project has been planned to complement the ISO Central Programme, which (besides some molecules such as H2O and H2) pays relative little attention to the molecular spectroscopy of the interstellar medium. The observations proposed here have been coordinated with other ISO proposals to study the emission of some other important molecules (C3, C4H2,H2O...) in the same sources. The results from these studies will provide a complete picture of these star-forming cores, and will be a major contribution to the IR spectroscopy of the interstellar medium.