We have measured the absorption and resonance Raman spectra in single crystals of the MX chain complex [Pt(en)(2)][PtBr2(en)(2)](SO4)(2) . 6H(2)O in order to study the mixed-valence state, the midgap state and the phase transition under various hydrostatic pressures up to 3.7 GPa at room temperature. Red shifts of both the intervalence charge-transfer absorption edge and the Pt-Br stretching mode Raman peak with increasing pressure show that the electronic state of Pt-II and Pt-IV in the mixed-valence state approaches that of Pt-III. An absorption band appearing above 2.5 GPa and a Raman peak appearing above 1.6 GPa suggest that a new phase coexists at pressures above 1.6 or 2.5 GPa. On the other hand, no enhancement of the optical absorption in the region of the midgap absorption bands (A-, B- and C-bands) is observed. This result shows that the midgap state is not generated by applying pressure at room temperature.