The partial pressure of CO2 (pCO2) and the concentration of dissolved CH4 and N2O have been studied in three estuaries of the southern Iberian Atlantic basin: Guadalquivir, Tinto - Odiel and Guadiana. A tidal cycle sampling of 24 h was carried out in the mouth of each estuary during July 2017. The tidal period in the three estuaries is semidiurnal and the tidal coefficient range between 0.67 and 0.75. Daily variations coupled to the tidal cycles in the three systems were observed. The three gases show the highest values and ranges of variation in the Guadalquivir (pCO2: 434.8 – 867.7 μatm, CH4: 12.0 – 74.0 nM, N2O: 8.5 – 14.0 nM). Changes in pCO2 values and CH4 and N2O concentrations were affected by several factors: temperature, salinity, exchange with the atmosphere, reactivity of the gases and various physical and biological processes taking place in the three estuaries. The highest concentrations were measured during ebb, which suggests that the estuaries export CO2, CH4 and N2O to the Gulf of Cádiz. The mean water - atmosphere fluxes of the pCO2, CH4 and N2O are positive, showing that the study area acts as a source of these gases to the atmosphere.