Abstract Poás Volcano (Costa Rica) resumed phreatic activity on 24 March 2006 after twelve years of quiescence. From March 2006 to June 2010, the initial phase of the ongoing eruption cycle, 110 phreatic eruptions were reported. This study presents the temporal variations in the chemical and isotopic (δD and δ 18 O) compositions of Laguna Caliente crater lake for the period prior to the eruption until June 2010. No systematic relationship with the phreatic eruptive activity exists. A combined mass and Cl budget analysis enables quantification of the seepage rate (7 kg s −1 ), the input rate of the ‘volcanic fluid’ ( Q f ), and the Cl concentration (Cl e ) in the evaporation plume (13 300 mg l −1 ). A modelling procedure for variable seepage rates leads to more realistic estimates of 50–100 kg s −1 , which better represent the observed lake water chemistry, hence suggesting dynamic fluid recycling between the lake and the underlying magmatic–hydrothermal system. The high Cl concentration in the evaporation plume and the dynamic fluid recycling at the lake bottom characterize Laguna Caliente as an ‘open-air’ fumarole, discrediting water chemistry as an efficient monitoring tool at the classic monitoring frequency. A conceptual model of phreatic eruptions is linked to our observations.