Corrosion of the cladding of spent Magnox nuclear fuel elements in water-filled storage ponds has produced a settled layer of fine particulate sludge (corroded Magnox sludge; CMS). CMS is believed to be predominantly composed of brucite (Mg(OH)2), and is contaminated with fragments of spent fuel, together with the associated Pu and fission products. For safe management of the ponds and eventual removal of the sludge as part of pond decommissioning, a comprehensive understanding of sludge chemistry is required. Brucite can sequester Pu from solution and, given that brucite can also form colloidal species in solution, this interaction may enhance Pu mobility within the ponds. To assess the likely effect on Pu mobility, we have measured the affinity of ‘dissolved’ 239Pu for a CMS simulant using a series of ultrafiltration experiments. The experiments were performed using a model Magnox storage pond liquor containing Pu, a CMS simulant, sodium carbonate, polyelectrolyte, and silica. Solution pH was controlled by the addition of sodium hydroxide, perchloric acid, and carbon dioxide gas to mimic storage pond and effluent treatment plant conditions. Multivariate analysis of the results was used to identify which parameters influenced Pu filterability. The presence of carbonate ions (0.03 M) had the largest effect on the system, allowing the majority of Pu to pass the filter. Addition of polyelectrolyte (0.35 ppm) produced a significant increase in Pu retention, even in the presence of carbonate and absence of CMS. The presence of CMS, even at only 3 ppm, increased Pu filterability, which was enhanced at higher pH. Over the pH range 7–11, Pu was generally better retained at higher pH. A 3-way interaction (the presence of CMS and polyelectrolyte at pH 11) affecting Pu filterability was also identified. The presence of silica (at 1 ppm) in the model system did not influence Pu filterability. Additional filtration experiments indicated a strong positive correlation between CMS concentration and Pu filter retention. This interaction was pH dependent, with more Pu filtered at pH 11.5 than at pH 10.8. Increased Pu filtration at high pH may reflect an increase in negatively charged surface species on the CMS particulate, and increased sorption of Pu although the precipitation of more CMS particulate at higher pH could also contribute. The pH dependence of Pu association with brucite is potentially an important control of Pu mobility within storage pond systems. The exact composition of the authentic pond sludge is unknown although it probably comprises mainly brucite, with smaller amounts of hydrous magnesium carbonate. Characterisation of the CMS simulant, synthesised from inactive Magnox swarf, showed the particulate to be primarily composed of platy brucite crystals, with a smaller but significant quantity of acicular artinite crystals.