Reflectance difference spectroscopy (RDS) has been performed during the growth and nitrogen-doping process of II-VI layers fabricated by molecular beam epitaxy (MBE). To the MBE chamber, equipped with an electron cyclotron resonance cell for N plasma generation, a RDS system has been attached via a normal incidence viewport which allows the acquisition of spectra during the doping process in the spectral range from 1.5 to 5.5 eV. ZnTe (001) surfaces have been studied under varying exposure conditions, like Zn, Te, and/or N plasma flux onto the sample surface. Furthermore, RDS features in the vicinity of the E-1 and E-1 + Delta(1) transitions were used to optimize online the doping performance of the N plasma cell by varying the source parameters, like N pressure and input power. Doping induced surface processes and surface saturation with activated N species have been investigated. Ex situ measured spectra are compared with it? situ acquired data to study the surface Fermi level pinning occurring at ambient pressures. Finally, in situ acquired RDS data of delta-doped ZnSe are presented.