Measurements of distribution of local velocities, flow angles and turbulence levels in arrays of louvred plate fins are presented in the Reynolds number range of 600 to 2300 corresponding to face velocity range of 0.45 m/s to 1.8 m/s. The measurements, using hotwire anemometry, were made in a louvre array model 16 x those found in typical air cooled compact heat exchangers. It was found that the 'traditional' methods for the calibration of hot-wires and the analysis of their signals were inadequate for this application. New techniques were developed and they are described in detail in this paper. The measurements showed that the mean flow angle gradually changed in the first few louvres to reach a constant fully developed value less than that of the louvre angle, and that the difference was a function of Reynolds number. The results show how the level of turbulence and its distribution vary with Reynolds number. They demonstrate that at Reynolds number values above 1000 the turbulence level in the flow becomes significant. The findings agree well with the qualitative flow visualisation studies in the literature.