Gold nanoparticles with an average diameter of approximate to 8 nm (Au approximate to 15 000) were irradiated with a tightly focused pulse laser at 355 nm in an aqueous solution of sodium dodecyl sulfate (SDS). Transient absorption spectra of the solution were measured at 25-100 ns after the laser irradiation. The observed transient absorption around 720 nm is assignable to the 2p <-- 1s transition of solvated electrons produced via multiple ionization of the gold nanoparticles. The nascent charge state of the gold nanoparticles was estimated from the transient absorbance. The dependence of the charge state on the SDS concentration shows a gradual increase from approximate to + 60 to approximate to + 70 in the 2 x 10(-4) to 3 x 10(-4) M range and an abrupt increase up to approximate to + 710 at the critical micelle concentration (CMC) of SDS, 8 x 10(-3) M. TEM measurements after laser irradiation reveal that the gold nanoparticles fragment into Au (approximate to 1000) at a SDS concentration of 3 x 10(-4) M, whereas they are significantly dissociated into Au (approximate to 100) above the CMC. The observed correlation between the nascent charge states and the extent of size reduction of the gold nanoparticles after the laser treatment indicates that the size reduction is caused by the Coulomb explosion of the highly charged gold nanoparticles. The mechanism of laser-induced size reduction is quantitatively discussed based on the liquid drop model.