Although the technique of Mossbauer spectroscopy is now 25-years old and widely used, little empirical work has been done to determine its accuracy in measurements on minerals. To solve this deficiency, two mineral standards (a grunerite and an almandine/andradite garnet mix) have been selected. Precision of the technique was measured through five different sets of experiments seeking to analyze the reproducibility of measurements on a single sample mount, on several identical mounts of the same sample, and on a set of mounts with different sample concentrations, run times, and background counts. The two mineral standards were analyzed by other scientists at seven different laboratories; their data were also fit by the MIT program. The standard deviation of multiple measurements on the MIT apparatus is better than 0.016 mm/sec for isomer shift, 0.060 mm/sec or better for quadrupole splitting, and 1.02% on individual peak area data. The standard deviation of interlaboratory measurements on the same minerals is slightly better because only ideal run conditions were used: 0.006 mm/sec for isomer shift, 0.023 mm/sec for quadrupole splitting, and 1.44% on individual peak area data. Probable errors on different aliquots of the same sample are approximately ±0.02 mm/sec for isomer shift and quadrupole splittings, and ±1.5% on area data for well-resolved peaks.