Spectrophotometric and spectrofluorimetric methods of analysis were conducted in dilute aqueous solutions of methylene blue and tetraphenylborate. The formation of double ion-pair was confirmed and its overall formation constant, ${Delta}H^{circ}$, and ${Delta}S^{circ}$ were obtained. The irradiation with wavelengths over visible region resulted in bleaching the mixed solution. The bleaching reaction was also proceeded at $55^{circ}C$. The reactions of the double ion-pair were briefly discussed.