The filtration properties of green liquor sludge were investigated. Three different experimental set-ups were used: one for low pressure (up to 3 bars), one for high pressure (up to 100 bars) and one for washing and lime mud addition experiments (1 bar). Green liquor sludge from four different pulp mills was investigated in the low pressure tests. The influence of pressure and the addition of calcium, magnesium and aluminium ions were investigated. The low pressure filtration tests showed that green liquor sludge is relatively compressible (the compressibility coefficient is approximately 0.6), and that the average specific filtration resistance could differ by one order of magnitude between different mills (1012-1013 m/kg). It was found that the addition of both aluminium and calcium ions improved the filtration properties, whilst the addition of magnesium has the opposite effect. The high pressure tests showed that a considerable amount of water could be squeezed out of the filter cakes at high pressures. The addition of lime mud to the green liquor decreased the average specific filtration resistance to a large extent. In the washing experiments, breakthrough curves were calculated. These curves showed considerable deviations from ideal displacement washing. The dispersion model was fitted to the experiments and a fair fit was obtained.