<p>Photochromic compounds can be classified into two categories. One is thermally irreversible (P-type) and the other thermally reversible (T-type). Although diarylethenes are generally P-type, a certain environmental modification such as the addition of specific chemicals can change it into the T-type. For example, the addition of Brønsted acid to a diarylethene possessing an amino group is known to change the reaction mode of photochromism. However, in order to restore its P-type photochromism, a scavenger of the acid is required. We have disclosed here that the P-type/T-type switching of diarylethenes can be achieved by the photochromic reaction of a coexisting spiropyran which can generate a strong acid by photoirradiation. When it takes the thermally stable merocyanine (mc) form, it works as a weakly acidic phenol. On the other hand, when it takes the spiropyran (sp) form generated by visible light irradiation, it works as a strongly acidic iminium sulfonate. Protonation to the accompanying diarylethene changes it from P-type to T-type. When the sp form thermally returns to the mc form, the P-type nature of the diarylethene is restored again. Thus, we have achieved the all-optical switching between the P-type/T-type photochromic reaction modes of diarylethenes. These procedures do not require the addition or removal of any agents from the photochromic system.</p> Graphical Abstract <p></p>