Bubble surface tension (BST) is measured by spinning single bubbles in a rotating cell and relating the resulting bubble deformation to the Centrifugal number Ce. Using this method bubbles in ''clean'' fresh water (distilled, deionized, passed through activated charcoal and filtered at 0.2 μm) are found to have a BST similar to that reported for the ''clean'' air-water interface (72.4 mN/m), whereas adsorption of organic films are observed to decrease the BST, with the effect increasing as the bubbles are aged. In natural coastal sea water (North West Arm, Halifax Harbour (10 m)) the BST decreases to 70.5 mN/m and in phytoplankton culture (Nitzschia pungens) decreases to 67 mN/m. Saturated stearic acid solution shows a BST of 57 mN/m. Coalescence of two bubbles with equilibrium surface film is accompanied by a rapid decrease in the surface tension, a decrease that we attribute to compression of the merging surface films at the newly created smaller interface. The decrease is most pronounced in water rich in organic surfactants. ¿ American Geophysical Union 1996