The focus of this work is the characterization of the order-disorder phase transition of calcium oxalate monohydrate. MDSC(TM), power-compensated DSC, EGA/MS, XRD, TG, and TMA were utilized to investigate the reproducibility, reversibility, and heating rate dependence of this transition. Flowing atmospheres saturated with water vapor were used to shift the dehydration equilibrium to a higher temperature so as to better resolve the order-disorder phase transition from the dehydration. Differences in thermal behavior of the sample due to the use of open versus semi-closed sample pans were also investigated. Correlations were made with spectroscopic (Raman, DRIFTS/MS, IR emission) and XRD results from the literature. This article reports the first MDSC(TM), DSC and TMA results for the order-disorder phase transition of CaC2O4 . H2O.