Acid fluids in geothermal reservoirs are rare. Their occurrence in geothermal systems associated with recent volcanism (Tatun, Sumikawa, Miravalles) probably indicates that the geothermal reservoir fluid was derived from volcanic fluid incompletely neutralized by reaction with feldspars and micas. Superheated steam containing HCl (Larderello, The Geysers) forms acid where it condenses or mixes with liquid at moderate temperatures (<300??C). The origin of steam with HCl is reaction of NaCl solid with rock minerals at high temperatures (>325??C). Cryptoacidity occurs at Los Humeros where HCl acidity is formed and neutralized without reaching the surface.