We study charge disproportionation in few quasi-two-dimensional BEDT-TTF-based compounds by following the temperature dependence of a charge-sensitive vibration ν₂₇ (B_{1 u}) of BEDT-TTF molecule. While in a charge ordered insulator θ-(BEDT-TTF)₂RbZn(SCN)₄ a difference between charge on the lattice sites is as high as 0.6e, a small charge disproportionation of 0.15-0.2e is found in two metallic compounds that become superconducting at low temperatures β^″-(BEDT-TTF)₂SF₅CH₂CF₂SO₃ and β -(EDT-TTF)₄ [Hg₃ I₈]_{(1 - x)}. In contrast to these, a pure metallic β^″-(BEDT-TTF)₂SO₃CHFSF₅ does not show any presence of charge disproportionation. This study suggests a correlation between a slight charge disproportionation in the metallic state and superconductivity.