⁹⁰Y microspheres are important therapeutic radiopharmaceuticals used in the treatment of liver cancer through a process known as selective internal radiation therapy. SIR-spheres® is a radiopharmaceutical product that is comprised of ⁹⁰Y microspheres suspended in sterile, pyrogen-free water for injection into patients. It is necessary to establish for the SIR-spheres® production the capability of accurately measuring the activity of this product to a traceable national measurement standard. An activity standard for SIR-spheres® was developed from a standard for ⁹⁰Y solution, employing a highly quantifiable chemical digestion process. Calibration factors for the manufacturer's ionisation chambers were determined for 1 and 5 ml of the SIR-spheres® product placed in Wheaton vials, for both 34% and 44% of ⁹⁰Y microsphere concentration.