Water electrolysis is carried out in both alkaline (25 and 2 wt.% KOH) and acidic (0.1N H2SO4) solutions for 8 s under a microgravity (mu-G) environment realized in a drop shaft. The effects of gravitational strength on gas bubble evolution behavior are analyzed in consideration of various factors (bubble movement, bubble assembly and single bubble). Under a mu-G environment, a collection of fine gas bubbles forms a froth layer in alkaline solutions, whereas bubbles frequently coalesce in acidic solution. Moreover, H-2 gas bubbles in alkaline jump from a cathode surface and O-2 bubbles often coalesce on an anode. In acidic solution both H-2 and O-2 bubbles frequently coalesce on electrode surfaces. Such gas bubble movements are reflected in the coalescence number and bubble residence time. A single bubble is characterized by the bubble size and the dynamic contact angle between a gas bubble and a Pt electrode, however, these factors are not essentially influenced by the gravitational strength. (c) 2005 Elsevier Ltd. All rights reserved. Keywords: Water electrolysis; Microgravity; Bubble