Atto Rho3B is a new fluorescent label related to the well-known dye Rhodamin B. The dye shows strong absorption and high thermal and photo-stability. The characteristic feature of the dye is its temperature dependent fluorescence yield. The dye is moderately hydrophilic. Atto Rho3B is a cationic dye. After coupling to a substrate the dye carries a net electrical charge of +1.