The adamantylation of diphenyl oxide and phenol with 1-chloro(bromo)adamantanes in a liquid phase has been studied. The reaction has been performed under kinetic control and under the conditions of reaching chemical equilibrium using the following catalysts: Amberlyst 36 Dry, in situ HBr, and AlCl3. The kinetics of isomerization of (1-adamantyl)- and (2-adamantyl)diphenyl oxides has been studied at 333 and 417 K in the presence of AlCl3 and Amberlyst 36 Dry, respectively. The equilibrium of the positional and structural “bridge–bridgehead” isomerization has been studied in the range of 333–523 K. It has been found that high selectivity of the adamantylation of diphenyl oxide or phenol can be achieved in the case of the use of sulfonated cation-exchange resins as a catalyst. In the presence of aluminum halides or HBr, the reaction mixture has been represented by all the possible isomers and adamantane.