The data of equilibrium decomposition pressure of bisphenol A - phenol adduct and saturated vapore pressure of bisphenol A and phenol solution were measured. Comparing the curves of the saturated vapor pressure of phenol and bisphenol A - phenol solution with the equilibrium decomposition pressure of bisphenol A-phenol adduct

it was found that the latter is higher than the former two. This discovery provided the experimental basis for direct dephenolization from the crystal adduct of bisphenol A-phenol using the solid decomposition reaction principle to obtain the bisphenol A whith high quality.