The partial pressure of CO₂ (pCO₂) and concentration of dissolved CH₄ and N₂O were studied during July 2017 in three estuaries of the southern Iberian Atlantic basin: the Guadalquivir, Tinto - Odiel and Guadiana. Two different sampling methods were adopted for each estuary: a longitudinal sampling along the course of the river, and a tidal cycle sampling of 24 h in the mouth of the estuary. In general, there is an increase in the concentration of gases towards the inner part of the river, in all three systems. Daily variations coupled to the tidal cycles were also observed 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. These three gases showed the highest value and range of variation in the Guadalquivir (pCO₂: 959 ± 1009 μatm, CH₄: 54.1 ± 74.5 nM, N₂O: 14.1 ± 16.1 nM), followed by the Guadiana (pCO₂: 595 ± 155 μatm, CH₄: 53.5 ± 62.9 nM, N₂O: 7.1 ± 0.5 nM), and the Tinto - Odiel (pCO₂: 581 ± 204 μatm, CH₄: 39.6 ± 43.5 nM, N₂O: 6.8 ± 0.3 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 estuaries object of study. The water - atmosphere fluxes of pCO₂, CH₄ and N₂O resulted positive, revealing that the study area acts as a source of these gases to the atmosphere.