A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is between 8.8% and 4.6%, depending on the wavelength and the solar zenith angle. The validation of the spectroradiometer was performed by repeated site visits to two European UV monitoring sites as well as by regular comparisons to the reference spectroradiometer of the European Reference Centre for UV radiation measurements (ECUV), in Ispra, Italy. The spectral solar irradiance measurements between the QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe through the development of a transportable unit) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the 10 intercomparison campaigns held between 2002 and 2004. If the differences in irradiance scales of up to 2% are taken into account, the agreement is of the order of 4% over the wavelength range 300 to 400 nm.