The results from an intercomparison of accurate mass measurement of a small molecule (molecular weight 475 Da) across a broad range of mass spectrometers are reported. The intercomparison was designed to evaluate the relative capabilities and the optimum methodology of the diverse range of mass spectrometers currently used to record accurate mass measurements. The data will be used as a basis for developing guidance on accurate mass measurement. The need for guidance has resulted from the continued growth in the use of accurate mass measurements for assignment of elemental formula in the chemical and biochemical industries. This has been fuelled by a number of factors and includes the rapid pace of instrument development, which has enabled accurate mass measurements to be made in a less costly, yet robust fashion. The data from the intercomparison will allow us to compare those protocols that produced excellent accuracy and precision with those that produced poorer accuracy and/or precision for each type of mass spectrometer. The key points for best practice will then be established from this comparison for each type of mass spectrometer and accurate mass measurement technique. A compound was sent to the participating laboratories (in the UK, Europe, and USA), the identity of which was not revealed. Each laboratory was asked to record a minimum of five repeat mass measurements of the molecular species using their local protocols and their preferred ionization technique or techniques. To the best of our knowledge there were no interfering (unresolved) ions that originated from the sample. A questionnaire was also completed with the experimental work. The information from the questionnaires was used to evaluate the protocols used to record the measurements. Forty-five laboratories have reported their results. To summarize the performance of mass spectrometers in the intercomparison, magnetic sector field mass spectrometers used in peak matching mode and FTMS reported the highest mean mass measurement accuracy (88 and 83%, respectively, achieved < or =1 ppm). Magnetic sector field mass spectrometers used in voltage scanning produced 60% of the mean mass measurements with accuracy < or =1 ppm. Magnetic sector field mass spectrometers used in magnet scanning modes, quadrupole-TOF and TOF instruments generally achieved mean mass measurement accuracy between 5 and 10 ppm. The two low resolution triple quadrupoles used in the inter-comparison produced mean mass measurement accuracy of <2 ppm. The precision of the data from each instrument and experiment type is an important consideration when evaluating their relative capabilities. Using both the precision and accuracy, it will be possible to define the uncertainty associated with the elemental formulae derived from accurate mass measurements. Therefore, a thorough statistical evaluation of the data is underway and will be presented in a subsequent publication.