The cohesive energy of metallic einsteinium determines whether einsteinium is a trivalent or divalent metal. The enthalpy of sublimation, a measure of the cohesive energy, is calculated from the partial pressures of einsteinium over an alloy. The partial pressure of 253Es has been measured over the range 470-870 K, using combined target and mass spectrometric Knudsen effusion techniques. An alloy was prepared with einsteinium dissolved in a ytterbium solvent to produce a very dilute solution. Partial pressure measurements on the alloy were amenable to the experimental technique and a data analysis using a Henry's law treatment of the data. Vapor pressure data are combined with an estimated crystal entropy S0298 and ΔC0p for ytterbium, to produce enthalpy, entropy, and free energy functions from 298 to 1300 K. The vapor pressure of einsteinium in a dilute einsteinium-ytterbium alloy is described by the equation log P(atm)=-(6815±216)/T+2.576±0.337, from which we calculate for the enthalpy of sublimation of pure einsteinium ΔH0298 (second law)=31.76 kcal/mol. The value of the enthalpy of sublimation is consistent with the conclusion that Es is a divalent metal.