The reaction of the indoles 1 with antipyrine‐4‐carbaldehyde 2 in the presence of HBF4 is investigated. The reaction is limited to reactive indole derivatives. The products contain two molecules of tetrafluoroborate. The antipyryl nucleus is O‐protonated. The structures of the cyanine dyes 3a–3d and 4 were elucidated by spectroscopic methods and elemental analysis.