We have performed thermodynamic calculation of the phase equilibria in the ternary monotectic system Al-Bi-Si. The liquid-liquid miscibility gap in the Al-Bi-Si system extends over almost the entire concentration triangle. The thermal analysis data for (Al0.345Bi0.655)(100-x) Si (x) alloys (x = 2.5, 5, 7.5, and 10 wt%) excellently agree with the calculated phase diagram. The experimental density difference of the coexisting liquid phases shows a good agreement with the density difference calculated in the approximation of ideal solution using the densities of pure elements and the compositions of L' and L'' from the thermodynamic calculation. The liquid-liquid interfacial tension in the (Al0.345Bi0.655)(100-x) Si (x) liquid alloys increases with Si content. The experimental temperature dependence of the interfacial tension is well described by the power low in reduced temperature (T (C)-T) at approach of the critical temperature with the exponent mu = 1.3, which is close to the value predicted by the renormalization group theory of critical behavior.