PURPOSE To construct an appropriate phantom for quality control use in diffusion-weighted imaging (DWI), to establish ground truth for measurement of apparent diffusion coefficient (ADC) and to characterize measurement linearity across a relevant physiological range of ADC. METHODS Aqueous solutions containing the polymer polyvinylpyrrolidone (PVP) were mixed at concentrations of 0, 10, 20, 30, 40 and 50% by mass PVP. These solutions were placed in 20 mL vials, arranged in concentric inner and outer circles, with a central water vial, and were fixed in a spherical phantom with a diameter of 194 mm, designed to fit into commercially-available MRI head coils. Two prototype phantoms were constructed, and underwent inter-site comparison in the US and EU. The phantoms were filled with an ice-water bath to ensure stable temperature; 0 °C temperature was verified by use of a thermocouple before and after scans. The phantoms were scanned using b-values of 0, 500 and 900 s/mm2 at several sites, using coronal and/or axial orientations and scan planes. RESULTS ADC values ranged from 0.12 to 1.12 × 10-3 mm2 /s, and exhibited a high degree of reproducibility across different scanners and imaging sites (coefficient of variations (CoV) ranged from 1.1 to 2.2% for 0 to 40% PVP, with 50% PVP at 11.3%). Little difference in ADCs was seen between inner and outer ring vials of the same PVP concentration (average CoV< 5% across vials, 10.3% for 50% PVP). CONCLUSION The range of ADCs covers a relevant physiological range, most notably in brain white matter. The ADCs of water vials were in excellent agreement with literature values of the diffusion coefficient of water at 0 °C (1.1 × 10-3 mm2 /s). The phantom provides a much needed quality control tool for DWI, and provides ground truth with the diffusion coefficient of water at 0 °C.