Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) continuous, small-scale activity erupting compositionally monotonous basaltic andesites that also dominate the entire, ~7000 year long, eruptive history. Only mineral zoning records reveal that basaltic andesites are the result of complex, open-system processes deriving minerals from a variety of crystallization environments and including the episodic injections of basalt. The condition of the mafic input as well as the generation of crystal-rich basaltic andesites of the recent, 1968–2010, and earlier eruptions were addressed by an experimental study at 200 MPa, 900–1,050 °C, oxidizing and fluid-saturated conditions with various fluid compositions [H2O/(H2O + CO2) = 0.3–1]. Phase equilibria were determined using a phenocryst-poor (~3 vol%) Arenal-like basalt (50.5−wt% SiO2) from a nearby scoria cone containing olivine (Fo92), plagioclase (An86), clinopyroxene (Mg# = 82) and magnetite (Xulvö = 0.13). Experimental melts generally reproduce observed compositional trends among Arenal samples. Small differences between experimental melts and natural rocks can be explained by open-system processes. At low pressure (200 MPa), the mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1,000 °C and high water activity. The residual melt at these conditions is basaltic andesitic (55 wt% SiO2) with 5 wt% H2O. The evolution to more evolved magmas observed at Arenal occurred under fluid-saturated conditions but variable fluid compositions. At 1,000 °C and 200 MPa, a decrease of water content by approximately 1 wt% induces significant changes of the mineral assemblage from olivine + clinopyroxene + plagioclase (5 wt% H2O in the melt) to clinopyroxene + plagioclase + orthopyroxene (4 wt% H2O in the melt). Both assemblages are observed in crystal-rich basalt (15 vol%) and basaltic andesites. Experimental data indicate that the lack of orthopyroxene and the presence of amphibole, also observed in basaltic andesitic tephra units, is due to crystallization at nearly water-saturated conditions and temperatures lower than 950 °C. The enigmatic two compositional groups previously known as low- and high-Al2O3 samples at Arenal volcano may be explained by low- and high-pressure crystallization, respectively. Using high-Al as signal of deeper crystallization, first magmas of the 1968–2010 eruption evolved deep in the crust and ascent was relatively fast leaving little time for significant compositional overprint by shallower level crystallization.