A review of the isotopic data from geothermal exploration areas in the Philippines indicates that the surface thermal waters and well discharges are enriched in heavier isotopes and deviate from the local meteoric water line defined by = (14 2). This enrichment is commonly due to: I ) near surface evaporation; and 2 ) mixing between meteoric waters and isotopically enriched deep magmatic waters, a phenomenon confirmed by deep well fluids in developed Philippine geothermal systems, such as Tongonan, Palinpinon and Alto Waters that are isotopically enriched due to the latter are found to be more significant to exploration because in most cases, these are representative fluids from hotter sources. The dominant process forming these waters can be distinguished by the relationship between stable isotopes and compositions.

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