Absorption, fluorescence, and fluorescence excitation spectra of two substituted [(5-methyl-2-pyridine-2'-yl-1,3-thiazole-4-yl)oxy]acetic acid and its methyl ester (2,2'-pyridylthiazoles) are studied at various pH values in aqueous solution. The acid exhibits pKa(1)=2.10+/-0.07 and pKa(2)=3.45+/-0.03, whereas the ester pKa=1.93+/-0.03. The protonation site is the pyridyl-nitrogen. When protonated, the cisoid conformer is the most stable; however, the transoid conformer is more stable in the deprotonated form. Fluorescence quantum yields close to unity are found. Large Stokes shift values are explained by the shortening of the inter-ring bond in the excited state. These compounds may be useful for metal sensing and as laser dyes.