The behavior of natural carbon dioxide (CO 2 ) droplets (8–10mm in diameter) were observed in a seafloor hydrothermal system at the Okinawa Trough. The natural CO 2 droplet contain 95–98% of CO 2 , 2–3% of H 2 S, and other gas species. The ascending CO 2 droplets were tracked by a remotely operated vehicle (ROV), and depth, temperature, salinity, pH and partial pressure of CO 2 (pCO 2 ) in seawater near the CO 2 droplets were measured during droplet ascent by a conductivity-temperature-depth sensor (CTD) and in situ pH/pCO 2 sensor. The visual images of the rising CO 2 droplets were recorded with a high definition television camera on the ROV. A mapping survey (400m×400m; 4 horizontal layers) revealed a dominant distribution of low pH area over the natural CO 2 venting site. The size and rise rate of CO 2 droplets decreased during their ascent in the water column from depths of 1424 to 679m (a tracking interval of 745m). The CO 2 droplets dissolved gradually to become small flakes of CO 2 hydrate while rising, and these ascending flakes were found to disappear at 679m depth. Although a pH as low as 5 was detected just above the liquid CO 2 venting site on the seafloor, no detectable pH depression in the water column ambient to the rising CO 2 droplets was observed. The results of the pH mapping survey showed only localized pH depression over the CO 2 venting site.