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A range of liquids suitable as quality control test objects for measuring the accuracy of clinical MRI diffusion sequences (both apparent diffusion coefficient and tensor) has been identified and characterized. The self‐diffusion coefficients for 15 liquids (3 cyclic alkanes: cyclohexane to cyclooctane, 9 n‐alkanes: n‐octane to n‐hexadecane, and 3 n‐alcohols: ethanol to 1‐propanol) were measured at 15–30°C using an NMR spectrometer. Values at 22°C range from 0.36 to 2.2 10⁻⁹ m²s⁻¹. Typical 95% confidence limits are ±2%. Temperature coefficients are 1.7–3.2 %/°C. T₁ and T₂ values at 1.5 T and proton density are given. n‐tridecane has a diffusion coefficient close to that of normal white matter. The longer n‐alkanes may be useful T₂ standards. Measurements from a spin‐echo MRI sequence agreed to within 2%. Magn Reson Med 43:368–374, 2000. © 2000 Wiley‐Liss, Inc.