An experimental study was undertaken to gain insight into the physical mechanisms that affect the laminar-turbulent transition process downstream of the leading-edge roughness condition. Sandpaper strips and small cylinders were attached to the leading edge of a heated test surface to simulate leading edge roughness typical of gas turbine blades. The roughness Reynolds numbers ranged from 2 to 2840. For free-stream velocities less than 5 m s(-1), the maximum roughness height was the primary contributor to deviations from the undisturbed case, irrespective of the roughness geometry. At higher free-stream velocities (5-7 m s(-1)), three of the rough leading-edge conditions induced a dual-slope region between the laminar and turbulent Stanton number correlations. Boundary layer measurements indicated that the first segment of the dual-slope was laminar, but the wall heat transfer significantly deviates From the laminar correlation. The second segment was transitional. The dual-slope behaviour and a waviness in the Stanton number distribution observed at higher free-stream velocities are believed to have been caused by nonlinear amplification caused by the finite disturbances at the leading edge.