ABSTRACT. We report the synthesis and full characterization of a family of phosphorus containing polymethine cyanines (phospha-cyanines). The compounds are easily prepared in two steps starting from readily available phosphanes. The impact of the P-substituents and the counterions on the structural and optical properties is investigated through a joint experimental/theoretical approach. Based on the study of the single crystal X-ray diffraction structures, all phospha-cyanines present a bond length alternation (BLA) close to zero, independently of the substituent and the counterions, which indicates an ideal polymethine state.