The photophysical and photochemical deactivation behavior of the homologous series of streptopolymethines 1,3-bis(dimethylamino)-tri-, -penta-, -penta-, -hepta- and -nonamethine has been investigated and is discussed on the basis of an energy scheme. The deactivation of the S1-states occurs by fluorescence and thermally activated photochemistry (E-Z isomerization). With the exception of the trimethine the fluorescence dominates at low temperature. The ab initio calculated potential curves of the S1 states indicate an increase in the rotation barrier with increasing length of the molecule. In the case of the nonamethine a multiple E-Z isomerization is discussed.