The delivery of suspended sediment from drainage basins has frequently been quantified in mass terms by use of the suspended sediment budget approach, which identifies sources, storage and output of mobilised sediment. This thesis investigates the particle size characteristics of the sediment associated with the key components of the suspended sediment budgets of four drainage basins in Devon, U. K. to determine whether particle size selectivity occurs in the delivery of suspended sediment from the hillslopes to the basin outlet. Attention focused on pasture land because previous studies had indicated that this was the dominant source of suspended sediment and that arable fields and channel banks were relatively insignificant in these catchments. Samples of sediment were mobilised from pasture hillslopes using a field-portable rainfall simulator; samples of suspended sediment were collected from the river channel during storm events either manually, by automatic pump samplers or by using rising limb siphon samplers; suspended sediment deposited on the channel bed was sampled using bed traps and by resuspending sediment deposited on the river bed during low flows; and sediment deposited on the floodplain during overbank flooding was collected using Astroturf mat traps or by sampling surface material. Samples were collected to investigate both temporal and spatial variability in grain size behaviour. All sediment samples were pretreated to remove organic matter and their chemically dispersed (absolute) particle size composition was measured using a Coulter LS 130 laser granulometer. The particle size composition of transported/deposited sediment was compared with that of the samples from potential sources to determine whether particle size selectivity had occurred. Where possible, measurements of the natural in situ particle size distribution (effective particle size) were also undertaken by quick return of samples to the laboratory for immediate measurement without pre-treatment using the laser granulometer. Particle size selectivity was found to have occurred in the mobilisation of sediment from the hillslope pasture land sources. Seasonal variations were identified in the particle size characteristics of both sediment mobilised from the hillslopes and suspended sediment samples. Spatial variations were identified in the particle size composition of sediment deposited on the floodplain. These seasonal and spatial variations reflect the particle size selectivity of detachment, transport and deposition processes which is in turn influenced by the aggregation or flocculation (effective particle size) of the sediment.