Mid-infrared spectra of neat n-heptane at room temperature are presented over a pressure range from ambient to 70 kbar. The application of hydrostatic pressure induces frequency shifts, band splittings, and significant changes in the line shapes of internal vibrational modes both in liquid and in solid phases. The results are discussed in terms of the liquid-solid phase transition and changes of the population of molecular conformers. Evidence for a solid-solid phase transition near 30 kbar is also presented.