We investigated the crystal hydrates of ammonium oxalate, potassium oxalate and oxalic acid as well as sodium oxalate. By taking advantage of the different crystal water contents we determined for the first time the relative ionization efficiency of water (RIE(H2O)) in an Aerodyne Aerosol Mass Spectrometer (AMS). The RIE is a key parameter for the quantitative measurement of a compounds' aerosol particle concentration with an AMS. Since the liquid water content of an aerosol particle determines its physical and chemical properties to a large extent, the knowledge of the water content is of high scientific interest. The investigation of the three salts led to a RIE(H2O) of 2. Application of this RIE(H2O) and further analysis of oxalic acid revealed two major characteristics, which need to be considered in future when handling oxalic acid. First, oxalic acid aerosol particles show a reduced crystal water content compared to the thermodynamically stable and expected value when produced under standard laboratory conditions. Second, solid oxalic acid and its solutions show significant contamination with ammonium when exposed to atmospheric air. These findings were verified by high resolution mass spectrometry and their implications for laboratory studies of hygroscopic growth and CCN properties of oxalic acid are discussed. (C) 2010 Elsevier Ltd. All rights reserved.