The effect of interaction between erosion processes has received only limited direct attention by researchers in the field of soil erosion by water. In modelling the processes involved in erosion it is uncertain whether it is justifiable to simply add the contribution of each separate erosion processes to give the total sediment concentration, or whether there may be some form of positive or negative interaction between flow-driven and rainfall-driven processes. Most laboratory experiments have shown that generally the rate of erosion in a rain-impacted flow is greater than for un-impacted flows of similar depth and velocity, indicating positive interaction. The experiments reported in this paper were carried out in the 5.8 by 1.0 m Griffith University Tilting Flume Simulated Rainfall (GUTSR) facility, using two different soil materials. Sediment concentration due to flow-driven and rainfall-driven erosion processes, or combination of both processes, depended on factors such as soil characteristics and streampower of the flow, and varied with time, tending to steady-state values. Interaction was defined as positive if the sediment concentration produced by both rainfall and overland flow acting together exceeded the sum of their individual concentrations. Such interaction was positive for a silty loam soil, but negative for a loamy sand. Reasons for the unexpected negative interaction are uncertain, but may be due to the suppression by rainfall of micro-rills or proto-rills which tended to form preferentially on the loamy sand under flow alone.