The Raman spectra of pure N₂, CO₂, and CH₄ were analyzed over the range 10 to 500 bars and from −160°C to 200°C (N₂), 22°C to 350°C (CO₂), and −100°C to 450°C (CH₄). At constant temperature, Raman peak position, including the more intense CO₂ peak (ν+), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature (PT) range studied. At constant pressure, the peak position for CO₂ and CH₄ increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N₂ first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N₂ is located between 0°C and 50°C at pressures above ~50 bars and is pressure dependent. Below ~50 bars, the inflection temperature was observed as low as −120°C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N₂, CO₂, and CH₄ to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature-dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N₂-CO₂-CH₄ gas systems in a future study.