An equilibrium shift equation for multistage complex systems was derived and used to explain the results of a plot of log Ai/(Amax-Ai) vs log[R] of the Bent method. The slope of the curve has integral values only when the values of [R] are within a definite range; this result was confirmed by a group of plots of log[b1[R]+b2[R] +... vs log[R] drawn by computer for a series of b. This equation was used to determine the complex ratio and overall stability constants for the extration of U and Th with ASA.

Key words: COMPLEX COMPOUNDS, COLOUR REAGENTS, STABILITY CONSTANT, COLOR REACTION, ARSENZO III, URANIUM COMPLEX, THORIUM COMPLEX, EQUILIBRIUM SHIFT METHOD