Abstnct The hydrogen content in alkaline water obtained from an electrolytic fiow cell was examined by measuring the diameter distribution of hydrogen microbubbl (hydrogen particles) in electrolyzed alkaline water using the dynamic light scattering (DLS) method. The influenoe of electrolysis conditions on the hydrogen content and the diameter distribution of hydrogen particles were also examined. The diameter of hydrogcn particles ehanges rapidly with elapsed time afler electrolysis, indicating that hydrogen partiel grow to form large particles. The distribution of the particle diameter shows two peaks. The mean diameter of hydrogen partiel is distributed mainly between 20 and 300 nm. The mean diameter decreases m'th an increasing current density up to 0.03 A dm 2. The diameter of hydrogen partiel is smaller than the cquilibrium diameter obtained by the concentration of dissolved hydrogcn, suggesting that the eloctrolyzed water is a transition state from supersaturated to saturated solutions. Hydrogen exists in particles as a oolloidal solution in a region of hydrogen content in electrolyzed water above 0.75 mM, which represents the saturation oonoentration of the dissolved hydrogcn. D 2001 Elsevier Science B.V. Al] rights reserved. Kejmvrds: Colloid; Dissolved hydrogen meter; DLS; Hydrogen evolution; Hydrogen partielcs; Supersatumtion.