A new photochemical reaction for mercury chemical/cold vapor generation (CVG) coupled to atomicfluorescence spectrometry (AFS) is described for the speciation analysis of inorganic mercury ion (Hg21) andorganic methylmercury (MeHg) in aqueous solution. The new CVG simply uses one reagent, formic acidonly, to react with Hg21 or MeHg in aqueous solution, under room natural light (Vis) or ultravioletirradiation (UV), for the generation of cold mercury vapor, which is subsequently detected by AFS. In thepresence of the UV, both Hg21 and MeHg can be converted to Hg0 for the determination of total mercury;and only Hg21 can be reduced to Hg0 with the Vis, thus determining Hg21 only. Then, the concentration ofMeHg can be calculated by subtracting the Hg21 concentration from the total mercury concentration. Theoptimal conditions for the best CVG efficiency are discussed, together with the interference from transitionmetals. There exists no significant interference from as high as 100 mg L_1 Co21 or Ni21, and 50mg L_1Cu21 for the determination of as low as 5mg L_1 Hg21. The new CVG minimizes the contamination sourcesand avoids off-line pre-oxidation of MeHg. A simple Hg21 standard series can be used for the calibration ofboth Hg21 and MeHg, eliminating the use of more toxic and more expensive MeHg standard series. Thelinear dynamic ranges of the calibration curves are up to 25mg L_1 with the UV and 300mg L_1 with theVis. The limit of detection is 0.003 or 0.2mg L_1 for total mercury with the UV or Hg21 with the Vis,respectively. The accuracy of this method was validated by determination of mercury in one certifiedreference water sample. The new CVG is a simple, fast, green, highly selective, and ultrasensitive yetinexpensive method for the speciation analysis of Hg21 and MeHg. It is expected to have similarapplications in other analytical atomic spectrometric techniques.