The single fibre collection efficiency is a key parameter to describe the filtration behaviour of unloaded fibrous filters, but it is and has always been difficult to achieve data about it experimentally. In a previous investigation [1] in which single collection efficiencies for a wide range of both biological and non-biological particle materials were measured under certain operational conditions, values much lower than those expected from model calculations were found. With special regard to this effect, in the present study, again under well defined conditions with even more effort and systematically excluding the remaining sources of possible errors, single fibre collection efficiencies in the range of critical stokes number St where bouncing effects occur were determined. A single fibre fixed in a special handling device was exposed to a well defined gas-particle stream. With an upstream optical particle counter; the velocity and the number of particles approaching the fibre was measured in situ. Together with the particle number collected on the fibre determined via CLSM micrographs, the single fibre collection efficiency was obtained in a very direct way. The results showed that beginning at critical St numbers (St > 0.8), single fibre collection efficiency, after having reached its maximum value of 22%, decreases significantly. Comparisons with published theoretical considerations gave remarkable disagreement. Common equations overestimate especially the adhesion efficiency by a factor of up to 6. This leads to much higher values for the single fibre collection efficiency at St > 1 than found here experimentally.