The reversible nature of the addition reaction of 1,3-benzoxazine and thiol at ambient temperature was discovered by investigating the reaction with using p-cresol-derived N-phenyl benzoxazine 1 and 1-octadecanethiol 2. The reaction was performed in several deuteriated media involving CDCl3 and CDCl3 + CD3OD, for monitoring their reaction by 1H NMR spectrometry. CDCl3 was a favorable solvent for the efficient progress of the reaction, and its combination with CD3OD allowed further acceleration of the reaction. In both cases, the reaction proceeded until conversion of 1 reached a certain ceiling value, to suggest that the reaction was reversible. This reversible nature was concretely confirmed by finding a dissociation reaction of isolated 3 into 1 and 2 in CDCl3. Analogously, a bisphenol A-derived bifunctional benzoxazine 4 and 1,6-hexanedithiol 5 underwent the polyaddition in CDCl3 + CD3OD at ambient temperature to afford the corresponding polymer 6. Successful depolymerization of 6 into small fragments was achieved by dissolving 6 in CDCl3.