The interstellar gas from which new stars form is crucially influenced by these same protostars: gas particles are excited and heated by the protostar. At the same time, the star formation process depends on the gas reservoir: if gas is too warm or dynamically ‘stirred’, it will not accumulate and contribute to the final mass of the forming star. This issue is specifically important in the formation of massive stars (more than eight times the mass of the Sun), because these stars produce at least 1000 times more radiation, impeding the accretion of gas. In this thesis I analyze primarily spectroscopic observations in the far-infrared and submillimeter wavelength regimes. Molecular constituents of interstellar gas are detected in these observations with ground- and space-based telescopes. Comparisons with model simulations provide insight into the influence of massive protostars on the surrounding molecular gas, both from a physical and from a chemical point of view. The results of this research indicate inhomogeneity in the density structure of the gas reservoirs that surround protostars, both in small-scale ‘clumpiness’ and on larger scales, for example in the form of axi-symmetric, carved-out areas through which excess energy can escape. Other aspects of the research emphasize how exchange between the gas phase and the surfaces of dust grains affects the interstellar chemical balance, and the power of the simple methylidyne molecule as a tracer of dynamically quiet gas in the otherwise violent environment of young massive stars. Chapters: (1) Introduction; (2) Star formation activity in IRDC G48.65; (3) Physical structure of the molecular envelope of AFGL2591; (4) Highly excited molecular gas in AFGL2591; (5) Chemical stratification in the Orion Bar; (6) CH gas toward NGC6334I; (7) Summary and future directions.