Four diffusion coefficients for one composition of the system H(2)O-Bu(4)NBr-HBr were obtained at 25 degrees C. The value of (D(12))(v) was negative and large as expected from the primary charge effect. However, the value of (D(21))(v) was also negative despite the fact that the primary charge effect predicts a positive value. This was interpreted to be a consequence of a large negative value for In y(2)/partial derivative C(1) which overshadows the primary charge effect.