A new arylhydrazone of b-diketone, (E/Z)-2-[2-(1-ethoxy-1,3-dioxobutan-2-ylidenehydrazinyl]ben­zenesulfonic acid (H₂L) has been synthesized by reaction between 1-ethoxybutane-1,3-dione and 2-sul­fo­phenyldiazonium chloride, and characterized by IR, ¹H and ¹³C NMR spectroscopies, ESI-MS and elemental analysis. H₂L exists in DMSO and H₂O solutions as a mixture of E- and Z-hydrazone isomeric forms and that an increase in the solvent polarity shifts the isomeric balance to the Z-hyd­razone form. Solvatochromic and photoluminescence properties of H₂L were studied. The ther­mal de­composition of H₂L was also investigated showing its high thermal stability with well-defined peak (552 K) of phase transition at 491−574 K