Three hemicyanine dyes containing N-(carboxyethyl)-2-methylbenzothiazolium group and bearing different substituent in p-position of phenyl ring of styryl moiety were readily prepared by a Knoevenagel condensation. Their UV/Vis and fluorescence spectra, aggregation, photostability and association with bovine serum albumin were studied. The maximum absorption and emission wavelengths of the dyes in different solvents were in the range 520–620 nm. Compared with a typical N-alkyl-2-methylbenzothiazolium hemicyanine dyes, the introduction of carboxyethyl group reduced aggregation and improved molar extinction coefficient, and photostability in water. These dyes may envisioning their potential usefulness for photodynamic therapy. The spectroscopic characterization of all the dyes synthesized is also described.