Close binary stars of Algol type consist of a main sequence star accompanied with a cool subgiant or giant, where former is now a more massive component. Apparently, this violates principles of the stellar evolution, and episodic mass transfer between the components has been postulated to explain this evolutionary paradox. In this short-lived process the initially more massive component has been converted into a low-mass giant, and layers which were originally deep within the star and have been altered by thermonuclear fusion during the stars’s main sequence evolution are now exposed. The surface chemical composition of both stars are precious diagnostic of the nucleosynthesis processes that occur deep within stars. Theoretical evolutionary models predict changes in abundances of the elements involved in CNO nucleosynthesis. Principal goal of present observational study was the determination of the abundance pattern in mass transferring binary stars. The two representative systems, archetype of a whole class, Algol itself, and its more massive counterpart, u Her, have been selected for detailed examination. High-resolution and high S/N time-series of spectra were secured with fibre-fed échelle spectrographs at the NOT, La Palma, Spain, BOAO, South Korea, and CAHA, Spain. These spectra were disentangled into individual component spectra of the components which eventually make possible detail atmospheric diagnostics including determination of the elemental composition. A number of auxiliary codes have been developed to facilitate analysis and estimate uncertainties of measured quantities. Algol is a hierarchical triple system in which the inner pair is partially eclipsing. This makes study of this binary quite challenging. For the first time the individual spectra of its components have been separated which eventually yield their complete characterisation. Their orbits and masses are revised which making possible determination of appropriate evolutionary models. The ratio of carbon and nitrogen is a very sensitive indicator of hydrogen-core CNO nucleosynthesis and a different mixing processes occurring before and after episode of masstransfer. In present work constraints on C/N have been found for the case of low and high mass Algol-type binary system.