Dilatometric measurements were used to determine phase transition temperatures and oxidation temperatures of Sr2MnO4-y, and several phases in the system SrMnO3-y. The use of dilatometry is based on differences among the molar volumina of the various phases determined by XRD. Oxidation of SrMnO3-y and Sr2MnO4-y, proceeds without change in the basic structure of perovskite and K2NiF4 type, respectively. Anion vacancies produce lattice expansion in reduced Sr2MnO4-y, and SrMnO3-y compared with Sr2MnO4.00 and beta-SrMnO3.00. In the case of SrMnO2.58, the ratio of lattice constants indicates narrow structural relations to beta-SrMnO3.