The existence of negative Maxwell-Stefan diffusivities is investigated
. For the case where the diffusion coefficients are taken to be compos
ition dependent, it is found that the theory of irreversible thermodyn
amics does not require all diffusivities to be positive definite. This
theoretical result is supported by literature diffusion coefficients
in electrolyte solutions, which are composition dependent, and by one
mixed-salt system which exhibits negative like-like ion diffusion coef
ficients. Further, an approximate method of obtaining like-like ion di
ffusion coefficients from tracer diffusivities is used to investigate
a further 35 salt systems. In most of these systems negative diffusion
coefficients do occur. The only exceptions are systems containing Hions and systems containing like ions of different valency. Finally, d
iffusion coefficients of a co-ion in an ion exchange membrane, which a
lso shows negative diffusion coefficients, are provided.