Abstract: It is shown that when ab ebulliometer operates steadily, the relation between the gravimetric concentration X10, and the concentration of the equilibrium liquid phase x1, is where K1 = y1/x1 is the phase equilibrium constant; r = ng/nl, the ratio of the vapor condensate reflux to the liquid one; a=(Ng-rNl)/N,Ng and Nl are the holdups of the vapor and the liquid respectively, and N is the total amount of the solution inside the ebulliometer. On this base, a rigorous formula for the infinite dilution activity coefficient γ∞ was derived: where H0=(r0 + a0)/(1+r0), the subscript "0" denoting x10 =0; The symbols in above expression was defined by C. A. Eckert, et al. A leaning ebulliometer was made as a modified Swietoslawski version with the Cottrell pump inserted in the insulating jacket. At an angle of 30°- 45° to the horizantal it operated steadily under the condition of r = 0 and a <0.005, so that | (x10-x1)/x1|+<0.5|K1 -1| %. The leaning ebulliometer is a very useful tool for measuring VLE (T-P-x) and γ∞.