This paper reports methodology and results of sedimentologic analyses performed on biogenic sediments collected during Ocean Drilling Program Leg 201 in the eastern equatorial Pacific (Sites 1225 and 1226). Particle size analyses were carried out with a laser particle sizer on both bulk and noncarbonate fractions to document changes in the relative proportions of the main biogenic, biocalcareous, and biosiliceous components of these sediments. The grain size distribution of these very fine grained sediments is generally polymodal and characterized by main modes at approximately 10, approximately 17, and approximately 40 mu m and minor modes at approximately 100 mu m and approximately 1 mm. The modes represent the common biogenic components of these pelagic sediments, including coccoliths ( approximately 2-10 mu m), pennate ( approximately 10-20 mu m) and centric diatoms ( approximately 20-50 mu m), radiolarian tests ( approximately 40-100 mu m), juvenile foraminifers ( approximately 40-50 mu m), fecal pellets (>50 mu m), and test and frustule fragments ( approximately 10-100 mu m). Downcore variations of particle sizes correlate with both small-scale and main lithologic changes in the sediment column and show pronounced shifts at unit/subunit boundaries at both eastern equatorial Pacific sites. Larger particle sizes characterize the units dominated by biosiliceous sediments. This relationship is particularly pronounced for the deeply buried diatom oozes (deeper than approximately 200 meters below seafloor) deposited during the late Miocene "carbonate crash". Conversely, the samples having the smallest grain sizes are concentrated in the coccolith oozes deposited during the late Miocene to Pliocene "biogenic bloom". Our results also show that changes of some of the key dissolved chemicals involved in anaerobic microbial respiration and methanogenesis coincide with changes of sediment textures. In particular, the concentrations of the two main by-products of microbial respiration at these sites, reduced Fe and Mn, increase with the coarsening of the mean diameter and mode in the mixed diatom and radiolarian biosiliceous sediments of Pleistocene to Pliocene age and in the deeply buried late Miocene diatom oozes.