RADIO observations using very-long-baseline interferometry (VLBI) can measure the deflection of electromagnetic radiation by the Sun's gravitational field with an accuracy of better than 1 milliarcsecond, and can thus be used to test General Relativity. For an object at an angle a from the centre of the Sun, the expected deflection is1 (1 + γ) (M s /r e )((l + cos α)/(l-cos α))1/2, where Ms is the mass of the Sun in geometrized units2 (1.477 × 105 cm), re is the distance from the Earth to the Sun in cm, and y is a parameter whose value is 1 if General Relativity is correct but which takes on different values in other theories of gravity. For γ = 1, the deflection is 1,750 mas at the Sun's limb, 4 mas at α =90° and 0 at α = 180°. Our analysis of ten years of VLBI data, including observations of objects in the range 2.5° < a< 178°, yields an estimate γ = 1.0002 with a formal standard error of 0.00096 and an estimated standard error of 0.002. This determination is comparable in accuracy and in good agreement with the determination from Mars–Viking time-delay measurements3.