This article discusses the results of wind tunnel measurements performed on a modified DU 97-W-300 aerofoil at Reynolds numbers between 1 × 106 and 10 × 106 in the cryogenic wind tunnel of DNW in Köln, Germany. The aerofoil was modified by reducing the trailing edge thickness from 1·74% to 0·49% of the chord. Although the measurements showed large scatter when flow separation occurred on the model, it was possible to establish the variation with Reynolds number of the maximum lift coefficient, the maximum lift/drag ratio and the design lift coefficient for a Mach number of 0·2. Furthermore, the effect of wrap-around Carborundum 60 roughness and zigzag tape of 0·4 mm thickness on the upper and lower surfaces was studied. The experimental results were compared with RFOIL calculations. The measurements indicate that there was no significant variation in the maximum lift coefficient with Reynolds number for the clean aerofoil. In contrast to the RFOIL calculations, the experimental maximum lift/drag ratio decreased with increasing Reynolds number from an average of 95 at R = 3 × 106 to about 85 at R = 10 × 106. The Carborundum 60 roughness had a larger negative effect on the aerofoil performance than the zigzag tape, but in both cases the aerofoil performance improved drastically with increasing Reynolds number. Copyright © 2004 John Wiley & Sons, Ltd.