The deviation from stoichiometry in Cu2-yO was measured as a function of oxygen partial pressure, 10(-13)less-than-or-equal-toP(O2)less-than-or-equal-to0.1 atm, and temperature, 873less-than-or-equal-toTless-than-or-equal-to1245 K, using a solid state electrochemical method. The measurements covered the whole width of the phase Cu2-yO for the temperature range 873less-than-or-equal-toTless-than-or-equal-to1245 K. The y-P(O2)-T relations are analyzed in order to identify the dominant point defects. It is found that in the high P(O2) range of the phase, the dominant defects are neutral copper vacancies, V(Cu)x. In the low P(O2) range there are two types of defects, dominating at different temperatures. At low temperatures the dominant defects are neutral copper interstitials, Cu(i)x. The analysis indicates that at temperatures higher than 1245 K the dominant defects are oxygen vacancies, V(O)x. The enthalpy and entropy for the formation of V(Cu)x and Cu(i)x are also determined.