This paper reviews X-ray emission from electrolyte aqueous solutions when irradiated by intense femtosecond laser pulses in air. First, some examples of X-ray emission spectra were shown when alkali halide aqueous solutions such as CsCl are used as samples. Dependence of X-ray emission spectra on laser intensity, solute atomic numbers, and solute concentrations are discussed on the basis of multiphoton ionization, resonance absorption, and so on. Laser chirp effects on X-ray emission were also discussed for understanding mechanisms occurring during a single laser pulse. X-ray intensity increase up to 10 3-10 4 times with prepulse irradiation was also introduced, and the mechanism of X-ray intensity increase was discussed from the viewpoint of laser ablation induced by the prepulse irradiation. Finally, preliminary results of time-resolved X-ray absorption fine spectroscopy using potassium iodide aqueous solution were presented.
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