The electrical conductivity of aluminum or chromium slightly substituted and slightly oxidized magnetites (0 < x < 0.27; 0 < δ 0.040) whose formula is Fe 3+[□ δFe (1−3δ) 2+Fe (1− x ) + 28 3+M x 3+]O 4 2− over the temperature range 300-77°K is affected by the composition of octahedral sites. In particular the Verwey transition temperature and its magnitude decrease with x and δ while the electrical conductivity and activation energy evolve differently on either side of this discontinuity.