The absorption spectra of two hemicyanine fluorescent dyes, namely, trans-4-[p-(N,N-di(2-hydroxyethyl))-amino-styryl]-N-ethyl pyridinium bromide (DHEASPBr-C2) and trans-4-[p-(N,N-di(2-hydroxyethyl))-amino-styryl]-N-octyl pyridinium bromide (DHEASPBr-C8), were studied at various levels of pH and high temperatures, and were then employed to dye cationic dyeable polyester. Their dyeing properties, fluorescent reflectance and colorimetric properties were explored. The novel fluorescent dyes existed in two forms of monocation and dication in solutions at low pH and high temperature. Overall, the influence of pH on colour depth and the maximum reflectance of dyed cationic dyeable polyester was extremely small. The adsorption mechanism of DHEASPBr-C2 and DHEASPBr-C8 on cationic dyeable polyester fibres was in good accord with the Langmuir type. Compared with DHEASPBr-C2, DHEASPBr-C8 exhibited comparatively faster adsorption rate, higher affinity and dye uptake, while its fluorescence shown by cationic dyeable polyester was slightly weak.