We estimated variation of reactive surface area (RSA) of minerals in the Galician (Spain) geothermal field using the chemical composition of fluids as input data. Our methodology is based on reconstructing the fluid composition according to a reaction progress schema that uses the fractional degree of advancement of the mass-transfer process. RSA of the principal mineral is estimated by using a transposed reaction rate that introduces experimental kinetic rate constants. We found that over the entire reaction process, RSA of feldspars and biotite varied by 2-4 orders of magnitude, thereby explaining the changes observed in CO2 partial pressure and fluid pH.