The results of our comprehensive isotope effect studies, in which seve
n different isotopically labeled (involving C-13, S-34 and H-2 labelin
g) BEDT-TTF derivatives and isotopically labeled anion [Cu((NCS)-N-15-
C-13)(2)](-) were utilized, are summarized. For the first time, convin
cing evidence for a genuine BCS-like mass isotope effect in an organic
superconductor is revealed in our studies.