Infrared reflectance and transmittance spectra (700 to 6500 cm - 1 ) of two charge transfer salts of the bis(ethylenedithio)tetrathiafulvalene, (ET) 2 Cu 5 I 6 and (ET) 3 Ag x I 8 , showing the properties of both electron conductors and solid electrolytes, were studied. Main spectral features of the room-temperature spectra of the (ET) 2 Cu 5 I 6 crystals strongly resemble those of the β-(ET) 2 X crystals (X=I 3 , AuI 2 , I 2 Br,PF 6 ), having similar arrangement of ET molecules in the conducting layer. However, temperature dependence of the (ET) 2 Cu 5 I 6 reflectance is found to be in strong contrast with those of metallic β-(ET) 2 X. Upon cooling, the interband transition exhibits a strong spectral weight loss and a shift towards lower energies, probably due to the formation of a gap on some part of the Fermi surface. The infrared spectra of (ET) 3 Ag x I 8 crystals are essentially different, despite similar molecular arrangement in the crystal structure. We suggest that it is mainly due to the essentially different band-filling factor. The positions of interference reflectance maxima show anomalies at temperatures 180 and 120 K confirming thus the existence of phase transitions in the (ET) 3 Ag x I 8 salt.