The relationship between surface bubble composition and gas flux to the atmosphere was examined at five large seeps from the Coal Oil Point seep field (Santa Barbara Channel, CA, USA). The field research was conducted using a flux buoy designed to simultaneously measure the surface bubbling gas flux and the buoy’s position with differential GPS, and to collect gas samples. Results show that the flux from the five seeps surveyed a total of 11 times ranged from 800–5,500 m3 day−1. The spatial distribution of flux from the five seeps was well described by two lognormal distributions fitted to two flux ranges. The seafloor and sea surface composition of bubbles differed, with the seafloor bubbles containing significantly more CO2 (3–25%) and less air (N2 and O2). At the sea surface, the mole fraction of N2 correlated directly with O2 (R 2 = 0.95) and inversely with CH4 (R 2 = 0.97); the CO2 content was reduced to the detection limit (<0.1%). These data demonstrate that the bubble composition is modified by gas exchange during ascent: dissolved air enters, and CO2 and hydrocarbon gases leave the bubbles. The mean surface composition at the five seeps varied with water depth and gas flux, with more CH4 and higher CH4/N2 ratios found in shallower seeps with higher flux. It is suggested that the CH4/N2 ratio is a good proxy for total or integrated gas loss from the rising bubbles, although additional study is needed before this ratio can be used quantitatively.