Photochromism is the phenomenon of a reversible, light-induced photochemical process that results in a color change of a chemical species. In the photoacoustic spectroscopic determination of photochromic species, the original spectral interference from weakly or non-photochromic compounds may be ruled out simply by the effect of photochromism in the analyteinduced by a light source. This new technique of separating the absorption characteristics of the analyte and interferences is named photochromic separation. In this report, photochromic separation is applied to the determination of Hg(II) dithizonate in the presence of Ag(I) or Pd(II) dithizonate. By the simple combination of an external light source and a laser photoacoustic spectrometer, the separation and determination of photochromic Hg(II) dithizonate can be performed simultaneously.