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Modeling a geothermal brine. CO2 in solution changes hugely when steam is separated. Calculated fugacity of CO2 using another program, entered fco2 value into React. Resulting output file is attached, mt43_03.txt, with the corresponding .rea file copied into it between the lines of **********. Took calculated concentration of CO2 in solution (including HCO3-) as reported in "Original basis - HCO3- " and "Elemental composition - Carbon" and replaced fco2 with that amount of CO2(aq). (Bicarbonate not specified as a separate species; pH is controlled by specifying OH- which represents total alkalinity of the brine.) This calculation gave a much higher pH value, even through Original basis and Elemental composition values are the same. Attached file is mt43_03c.txt Only red flag I see is a negative value for "Original basis - H+" in the latter calculation. This looks like a NASTY error in React. Is there a patch or easy work-around ? I have had similar problems using specifying values of fugacity of H2S. mt43_03.txt mt43_03c.txt