Abstract. The variation of partial pressure of CO2 (pCO2), 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 pCO2 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−2 d−1 in dry periods to 245.3.1 ± 51.5 mmol C m−2 d−1 following periods of heavy rainfall. The increased inputs of pCO2 following periods of high rainfall are believed to be derived from increased inputs of pCO2 from groundwaters to the lakes.