Many pumped rainfall simulators used in soil erosion studies use pulsed rain to control the rainfall intensity. We examined the effect of the rain pulsing on sediment concentration and size using three different pulse cycles with the same rainfall intensity. There was considerable variation in sediment concentration through the pulse cycle: the highest concentration was up to four times that of the lowest concentration. Furthermore, the particle size distribution also varied: the peak median particle size was double the lowest median particle size. The magnitude of differences in sediment concentration and particle size were greater the longer the pulse cycle and these dynamics will vary between rainfall simulators and studies. We suggest the impact of the pulsing on sediment is significant and should be investigated prior to experimentation so that sampling periods are designed to avoid bias introduced by fine temporal scale sediment dynamics. Copyright © 2009 John Wiley & Sons, Ltd.