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The variation of partial pressure of CO₂ (<i>p</i>CO₂), pH, salinity and dissolved organic carbon (DOC) in surface waters of 12 coastal Brazilian lakes was examined following periods of contrasting rainfall. Periods of high rainfall were followed by a large, almost 10 fold, increase in <i>p</i>CO₂ and a one unit decrease in pH in the lakes, whereas no consistent changes in DOC were observed. CO₂ emissions to the atmosphere from the Brazilian coastal lakes studied here were highly enhanced, on average, from 28.5 ± 6.0 mmol C m^&minus;2 d^&minus;1 in dry periods to 245.3.1 ± 51.5 mmol C m^&minus;2 d^&minus;1 following periods of heavy rainfall. The increased inputs of <i>p</i>CO₂ following periods of high rainfall are believed to be derived from increased inputs of <i>p</i>CO₂ from groundwaters to the lakes.