Fibrous filters, produced from binary mixtures of glass fibres of two diameters and a range of proportions, have been tested with submicrometre-sized monodisperse aerosols. The results have been interpreted with the help of pressure drop calculations based on a variational Stokes flow model. The theory predicts that the pressure drop across a mixed-fibre filter is similar to that of a single component filter with the same packing fraction, composed of fibres with the surface area mean diameter of the mixture, and the experimental results verify the calculation. The filtration efficiency is not treated by rigorous theory, but power law fits show capture by interception to have a stronger dependence on fibre size than capture by diffusional deposition, though both relationships are weaker than theory predicts. This aspect of behaviour, along with the position and shape of the maximum penetration peak, also shows reasonable agreement with theory for a uniform filter composed of fibres with the surface area mean diameter of the mixture. Crown Copyright