We present electrical resistivity (p) and magnetic susceptibility (j) data of the Sr2FeMoO6 double perovskite. We found that the as prepared sample shows semiconducting p(T) in the whole temperature range and a magnetoresistance behavior typical of electron tunneling at the grain boundaries (gb). When the oxygen of the gb is removed p drops one order of magnitude. Below the magnetic ordering temperature Tc 405 K and above 590 K. this sample is metallic (dp/dT > 0), while for Tc < T < 590 K we observe a weak localization. The x(T) data follow the Curie-Weiss law only in a short temperature range above Tc.