Simulation of particle size evolution and of scale formation and corrosion along the flow path is of the greatest importance for data interpretation as well as for engineering applications (eg minimisation of the scale formation) in geothermal and hot water plants. An equilibrium simulation algorithm and a computer program for calcite scale formation in a CaCO3-H2O-CO2 system has been developed using the Davies and Fitter activity calculation methods. An equilibrium simulation algorithm and a computer program for CaCO3, CaSO4, BaSO4, SrSO4 scale formation in a Na-K-Mg-Ca-H-Ba-Sr-Cl-Br-SO4-OH-HCO3-CO3-CO2-H2O system are also constructed using the Davies and the Fitter activity calculation methods. The program determines the Langelier and Ryznar saturation indexes too, which are useful for the description of the scaling and corrosion characteristics in a solution. An equilibrium simulation algorithm and a computer program for the determination of the bubble point and of the partial pressure profiles of the gases CO2, CH4 and N-2 between the,bubble point depth and the well head in a geothermal well are constructed.