The influence of Cu contamination on radiation-induced defect reactions in n-type Czochralski-grown silicon (Cz-Si) crystals has been studied by means of the Hall effect technique, deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS with supporting theoretical modeling of defects. It is found that the contamination of Cz-Si samples with Cu does not influence significantly the energy spectrum and introduction rates of the principal electrically active defects induced by electron irradiation. The vacancy-oxygen (VO) centre, divacancy (V2) and a complex consisting of a silicon self-interstitial with the oxygen dimer (IO2) are found to be the dominant radiation-induced defects in Cu-contaminated samples as well as in uncontaminated ones. An isochronal annealing study has shown that the presence of Cu affects the annealing behaviour of the vacancy-related defects. In Cu-doped samples the VO centre disappears upon annealing at significantly lower temperatures (175-250°C) compared to those of the VO disappearance in the uncontaminated samples (300-375°C). The disappearance of the VO centres in the Cu-doped samples occurs simultaneously with an anti-correlated introduction of a defect with an energy level at about Ec- 0.60 eV. It is suggested that this defect is formed by the interaction of a mobile Cu atom with the VO complex. According to results of quantum-chemical modelling, in the most stable configuration of the Cu-VO defect a Cu atom occupies a tetrahedral interstitial position nearest to the elongated Si-Si bond of the VO centre. The presence of the Cu atom is found to result in the further elongation of the Si-Si bond and a shift of the VO acceptor level to the middle of the gap. The annealing behaviour of V2 has also been found to be different in the irradiated Cu-doped samples compared to that in the uncontaminated ones. The most probable reason for this difference is an interaction of mobile Cu atoms with di-vacancies. An energy level at about Ec-0.17 eV has been tentatively assigned to a complex consisting of a Cu atom and a di-vacancy.