The role of substitutional ions (Ca2+) in colossal magnetoresistent systems such as La1-xCaxMnO3 has been usually restricted to that of supplying electronic holes (Mn4+) to the system. New experimental evidence is reported showing that accurate control of cationic and anionic vacancies in slightly doped manganites also influences on the hole concentration. (La0.95Ca0.05)(0.965)Mn0.965Oy samples have been prepared under different atmospheres in order to control the oxygen content and the average Mn oxidation state. The variation of the oxygen content strongly influences the magnetic properties. In absence of Mn4+, antiferromagnetic behavior is observed with a canted spin structure. The increase of electronic holes leads to ferromagnetic behavior. (C) 2004 Elsevier B.V. All rights reserved.