Using the velocity analyzer of Zartman with improved technique the combined velocity spectrum of Bi atoms and Bi 2 molecules was obtained at 827°, 851°, 875°, 899°, 922°, 947° C. From the spectral distribution curves the relative abundance of Bi atoms and Bi 2 molecules in the beams at the above temperatures could be determined to 1 per cent. The vapor pressure curve of Bi was obtained experimentally by the method of effusion and the values so obtained were combined with the degree of dissociation of the vapor as computed from the beams to give the heat of dissociation. The heat of dissociation was computed from the data, assuming the pressure to be given by the temperature of the crucible T c . In calculating the heat of dissociation, the equilibrium temperature was taken as that of the slit chamber T s which was 24° above T c . The results of these calculations plotted with log 10 K p as ordinates against 1 T s give a straight line whose slope yields the value of the heat of dissociation as 77,100±1200 calories. The curves for the distribution of velocities observed and computed on the assumption of a given ratio of Bi atoms to Bi 2 molecules in the beam were compared in an attempt to test the law of distribution of velocities. On the high velocity side agreement in two curves was obtained within the limits of experimental accuracy. On the low velocity side important deviations were noted of such a sort that the observed curves below a velocity α 2 , (α is the most probable velocity) gave more molecules than the theory demanded. Other deviations were observed on some of the runs taken with a fourth slit in which a deficiency of molecules was observed between velocities of .75α and α 2 . This deviation was probably due to a warping of the fourth slit carriage due to heat. The nature of the variation at velocities less than α 2 indicated the presence of molecules of greater mass than Bi 2 in the beam and at the lower temperatures a distinct peak corresponding to Bi 8 molecules was observed which were present to less than 2 per cent. The vapor pressure curve for Bi was determined by least square reduction of the observed points to be given by log 10 P = − 52.23 × 195.26 T + 8.56 between 1100° and 1220° abs. It lies very close to the extrapolated curve given in the International Critical Tables.