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 CaCO 3 H 2 OCO 2 system has been developed using the Davies and Pitzer activity calculation methods. An equilibrium simulation algorithm and a computer program for CaCO 3 , CaSO 4 , BaSO 4 , SrSO 4 scale formation in a NaKMgCaHBaSrClBrSO 4 OHHCO 3 CO 3 CO 2 H 2 O system are also constructed using the Davies and the Pitzer 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 CO 2 , CH 4 and N 2 between the bubble point depth and the well head in a geothermal well are constructed.