Intense chemical weathering accompanies physical instability at the site of a repeatedly active landslide in North Derbyshire. In order to describe and quantify the chemical weathering, a programme of water sampling and analysis was devised. A sequence of reactions are proposed, based on theresults of this work, to account for the observed concentrations of chemical species in the drainage waters. It is thought that pyrite oxidation, accelerated by the presence of catalytic bacteria, is responsible for the considerable acidity of these waters. Additional reactions involving carbonates and silicates occur at strictly comparable rates and consume over 99% of the acidity prior to the water's emergence in a number of ochre-precipitating springs. Analysis of the solid reactants and products confirms the suggested sequence of events and suggests a number of ways in which chemical weathering might be related to slope stability. Clay minerals appear to be little affected by weathering, and the growth of precipitate minerals such as gypsum in joints and on bedding planes might be a more important mechanism in shale breakdown. After this initial rapid physical disintegration, chemical weathering, at the surface, proceeds relatively slowly. Chemical processes build up stresses within the rocks and possibly help to maintain any inherent planes of weakness. Ultimately landslide movement is triggered by increases in porewater pressures brought about by fluctuations in local ground water levels.