Measurements of the coexistence curve of the binary-fluid mixture isobutyri
c acid + water with added KCl salt at various concentrations: show an impor
tant dependence on the critical temperature and the critical exponent beta.
The phase-transition region shifts as a function of the salt concentration
. The effective -critical exponent beta* = (0.266-0.293) is neither compati
ble with the Ising value beta = 0.325 nor with the Fisher renormalized valu
e similar to 0.365.