Calcium diglyceroxide has been reported as a very active phase in the transesterification of triglycerides with methanol for biodiesel production. This work reports on the determination of the crystal structure of Ca diglyceroxide by Patterson-based direct methods from synchrotron X-ray powder diffraction (XRD) data in combination with characterisation by Differential Thermal Analysis-Thermogravimetry (DTA-TG), Scanning Electron Microscopy (SEM) and X-ray Photoelectron and Fourier Transform-Infrared spectroscopies (XPS and FT-IR). Its crystal structure is formed by molecular tetramers held together by a complex H-bond network. Both the XRD structural determination and the O1s XPS core level indicate the presence of a basic non-protonated O- anion at the surface of Ca diglyceroxide. Along with the presence of surface lipophilic CH units, this O- anion may be the origin of this molecule's high activity relative to CaO. (C) 2012 Elsevier Inc. All rights reserved.