<p> A Au@Ag core–shell nanoparticles (NPs)-based digital image colorimetric method, coupled with headspace single-drop microextraction technique (HS-SDME), was developed for the sensitive speciation of sulfite and sulfide. The entire experiment was conducted in a compact headspace bottle with Au@Ag NPs as droplets that selectively reacted with H₂S to generate a&#xa0;brown product, prior to the readout of RGB values through a smartphone, thus to conduct the accurate qualitative and quantitative determination. In this way, the speciation differentiation of sulfite and sulfide was achieved by controlling their respective conditions of vapor generation. In addition, by combining the sample introduction of chemical vapor generation, headspace enrichment, and drop microextraction, HS-SDME effectively alleviated matrix interference and further improve analytical performance. The linearity ranges for sulfide and sulfite were 4–14 and 8–18 μmol·L⁻¹, with detection limits (LODs) of 1.60 and 1.32 μmol·L⁻¹, respectively. This whole device featured a simple structure and compact size, requiring 3 mL of sample volume, which replaced large-scale chromatographic separation equipment with a smartphone application to some degree, enabling detection to be completed in 30 min. This method demonstrated promising application prospects in on-site speciation determination of sulfite and sulfide in environmental water.</p> Graphical abstract <p></p>