AbstractThe absorption spectrum ofp,p′‐bis‐dimethylamino‐azobenzene has been measured in solvents of different acidities. The bathochromic shift of the long‐wave transition associated with protonation of the azo link falls in line with the corresponding shifts, observed for a series ofp′‐substitutedp‐dimethylamino‐azobenzenes. The same is true – inside wider limits of error – for the pK′ values of the equilibrium: base + H⊕⇆ azonium‐cation of these compounds. These results support the view that protonation ofp,p′‐bis‐dimethylamino‐azobenzene does not differ in principle from that of the unsymmetricp