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Abstract Background The T 1 Mapping and Extracellular volume (ECV) Standardization (T1MES) program explored T 1 mapping quality assurance using a purpose-developed phantom with Food and Drug Administration (FDA) and Conformité Européenne (CE) regulatory clearance. We report T 1 measurement repeatability across centers describing sequence, magnet, and vendor performance. Methods Phantoms batch-manufactured in August 2015 underwent 2 years of structural imaging, B 0 and B 1, and “reference” slow T 1 testing. Temperature dependency was evaluated by the United States National Institute of Standards and Technology and by the German Physikalisch-Technische Bundesanstalt. Center-specific T 1 mapping repeatability (maximum one scan per week to minimum one per quarter year) was assessed over mean 358 (maximum 1161) days on 34 1.5 T and 22 3 T magnets using multiple T 1 mapping sequences. Image and temperature data were analyzed semi-automatically. Repeatability of serial T 1 was evaluated in terms of coefficient of variation (CoV), and linear mixed models were constructed to study the interplay of some of the known sources of T 1 variation. Results Over 2 years, phantom gel integrity remained intact (no rips/tears), B 0 and B 1 homogenous, and “reference” T 1 stable compared to baseline (% change at 1.5 T, 1.95 ± 1.39%; 3 T, 2.22 ± 1.44%). Per degrees Celsius, 1.5 T, T 1 (MOLLI 5s(3s)3s) increased by 11.4 ms in long native blood tubes and decreased by 1.2 ms in short post-contrast myocardium tubes. Agreement of estimated T 1 times with “reference” T 1 was similar across Siemens and Philips CMR systems at both field strengths (adjusted R 2 ranges for both field strengths, 0.99–1.00). Over 1 year, many 1.5 T and 3 T sequences/magnets were repeatable with mean CoVs