Colloidal-size particles play an important, yet not very well understood, role in the speciation of phosphate in freshwaters. This study assesses the size distribution of molybdate reactive phosphorous in various 1.2 μm-filtered freshwaters using cross-flow filtration. Concurrently, colloid size was determined by a single particle counter technique, and mineralogical composition by a combination of transmission electronic microscopy coupled with energy dispersive spectroscopy and selected area electron diffraction. The percentage of colloidal molybdate reactive phosphorous varied a great deal in the samples studied: it ranged from 26% down to only 3%. A direct comparison of the percentages of colloidal-P can be misleading. The simultaneous determination of the amount, size distribution and mineralogical composition of the colloids present in the water bodies proved to be essential for interpreting the results. The causes of the observed variation were numerous and were site-dependent.