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Water–bentonite suspensions behave as non-Newtonian fluids with exceptional rheological and filtration characteristics at low temperatures which deteriorate at temperatures higher than 1200C. Additives restore these characteristics but many of them are thermally unstable at the temperatures encountered, for example in oil-well and geothermal drilling. Greek lignite has been proven to be an excellent additive for water–bentonite suspensions at temperatures up to 1770C. In this work we attempt to assess the reason for such good performance by studying the surface characteristics and the permeabilities of filter cakes of water–bentonite suspensions with and without the additive (various lignite types) after exposing the samples to thermal static aging at 1770C for 16 hours. The filter cakes are produced with an American Petroleum Institute filter press allowing filtration for sufficient time to produce a filter cake with adequate thickness. The surface morphology of the filter cakes has been assessed with a scanning electron microscope. The permeabilities of the filter cakes were determined with an in-house technique which creates a ‘wet core’of the filter cake of sufficient thickness and the water permeability is measured in a Hassler type meter. The differences between the reference samples (cakes from bentonite suspensions at room temperature) with cake samples from thermally aged water–bentonite suspensions and water–bentonite–lignite suspensions both in surface characteristics and in cake permeability are noted and discussed.