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Reactor physics, thermal hydraulics or material sciences are mainly studied to understand phenomena occurring in nuclear power plants or to improve the performance of existing or future reactors: improvement of core performances, research of new materials (fuels, core, reactor pressure vessels, internal structures, …) and so on. Nevertheless, in the same way as these physical sciences, chemical sciences (solution chemistry, thermodynamics, kinetics, ...) can also contribute to these objectives.Firstly, to improve availability, safety and lifetime of existing reactors, the best chemical conditioning has to be used in the different nuclear reactor circuits (primary, secondary, tertiary and auxiliary circuits). In that way, solution pH (B-Li coordinated chemistry), redox (value and chemical choice), water treatment, on-line chemical automates have, for example, to be optimized. Secondly, chemistry allows understanding phenomena occurring in the different reactors circuits. In the primary circuit, the understanding of the contamination by corrosion products (Fe, Ni, Cr, …), activation products (58 Co, 60 Co, …), fission products and actinides is a crucial issue for reactor operation and design. The main processes involved in the contamination transfer are dissolution/precipitation, erosion/deposition, convection, purification, neutron activation, radioactive decrease. Consequently, the chemistry knowledge has a role to play in the same way as other sciences (nuclear physics, material science, thermohydraulics, …). In the secondary circuit, formation of concentrated media, which can lead to the tube fouling and sometimes to the tube support plates blockage of steam generators, is of major concern. These phenomena have for consequences a loss of thermal performance and efficiency of SGs. Chemistry, in addition to thermal hydraulics, can help to explain them and consequently to their mitigation by, for example, optimization of the secondary side chemical conditioning (pH level, amine choice, …). In the tertiary circuit, scale, corrosion, deposits and microbial growth affect the plant performance and can be partially controlled by a “good” chemistry.To understand the chemistry role or impact, an important R&D work is necessary. Indeed, only few data exist in the literature for the physico-chemical conditions met in a nuclear reactor. So these data have to be obtained either experimentally (solubility measurements, liquid-steam equilibrium study, solid solution interfacial flux measurements and so on) or by extrapolation (which implies the development or the use of theoretical models (for example, for temperature or ionic strength effects)). Then, chemical database have to be built and simulation codes (for example, to describe cold shutdowns or SGs tube fouling) have to be developed. These R&D studies are either home-made or realized in collaboration with many companies such as EDF, AREVA NP or GDF SUEZ.The goal of this paper is to present the CEA methodology used for these R&D studies and few applications of chemistry to the understanding of some phenomena occurring in water-cooled nuclear reactors.