We report the results of high-pressure Raman scattering studies of the cubic andmonoclinic polymorphs of tetracyanoethylene (TCNE). The evolution of the Ramanspectrum at high pressures suggests that the cubic form is stable up to about8 GPa. Subsequent pressurization leads to a gradual loss of transparency, and thesample becomes opaque to visible light above 14 GPa. In the monoclinic samples,qualitative changes are observed in the Raman spectrum above 3.6 GPa whichindicate a subtle phase transition around this pressure. These changes are reversiblewhen the pressure is reduced from peak values of about 4.5 GPa. At still higherpressures, the sample progressively becomes black, similar to what is observed incubic TCNE. The Raman spectrum of the sample above 7 GPa is indicative ofpolymerization of TCNE. The spectrum of the pressure cycled opaque phaseshows broad features characteristic of an amorphous phase, which is understoodas being due to random cross-linking of TCNE in the pressure-reducing cycle.