In the compound BaMg2Si2O7, the Mg2+-ions can completely be replaced by Co2+-ions. The resulting solid solutions show a phase transition from a low to a high temperature phase. Both, the thermal expansion behavior as well as the phase transition temperature change significantly with the Mg/Co-ratio. The phase transition runs parallel to an increase in volume, which can be detected using dilatometry. The introduction of small CoO concentrations leads to an increase in the phase transition temperature. In order to determine the thermal expansion behavior, dilatometty, as well as high temperature X-ray diffraction was used. The substitution of MgO by up to 50 mol% CoO does not lead to a significant change in the thermal expansion behavior. The measured coefficients of thermal expansion lie in a range between 7.0 and 24.0 x 10(-6)K(-1). (C) 2015 Elsevier B.V. All rights reserved.