Abstract We report on room-temperature Raman measurements on κ-di[bis-ethylenedithiotetrathiafulvalene] copper dicyanamide bromide (κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br) and the isotopic analog 13 C(6)-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, in which the six central carbon atoms in the inner rings of the BEDT-TTF molecule are replaced with 13 C, and assign the features to the symmetric intramolecular modes of vibration, by comparison with a recently published study on neutral BEDT-TTF. We then used these data to help us make a more definitive assignment that has been previously possible of the features in the low-temperature IR conductivity spectra of (BEDT-TTF) 2 Cu[N(CN) 2 ]Br. It was also necessary to measure the low-temperature IR conductivity of additional isotopic analogs of BEDT-TTF, namely those containing 13 C(6), 13 C(2), 34 S(8) and d(8). We found, as expected, that the spectra are dominated by the electron-phonon activated totally symmetric (A g ) modes, but also that the nature of the strongest of these, ν 3 (A g ), is changed dramatically in (BEDT-TTF) 2 Cu[N(CN) 2 ]Br from what it is in neutral BEDT-TTF. We also found, unexpectedly, that one of the stronger IR features is most likely due to an activated non-totally-symmetric (B 3g ) mode, instead of a normally IR-active mode, which had been previously assigned, and further that several other (B 3g ) modes appear to be activated in the IR spectra.