Abstract Water electrolysis is carried out in both alkaline (25 and 2 wt.% KOH) and acidic (0.1N H 2 SO 4 ) solutions for 8 s under a microgravity (μ-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 μ-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.