Abstract: The relation between the gravimetric liquid concentration (χ_(10)) and equilibrium one (χ_1) is developed for the system within an ebulliometer or a still operating steadily. From the relation, the necessary and sufficient condition for χ_(10)→χ_1, and a new equation for calculating limiting activity coefficient (γ~∞) are deduced. This equation proves more accurate than that proposed by previous researchers. The leaning ebulliometer, a new apparatus, is designed to measure bubble points and γ~∞without composition analysis, its operating line (T, P, χ_(10)) proves a good approximation to the bubble-point line (T, P, χ_1): |(χ_(10)-χ_1)/χ)|<0.005|K_1-1|, where K_1=y_1/χ_1 is the K-value.