Causes for slow weathering and erosion in the steep warm monsoon subjected Highlands of Sri Lanka In the Highlands of Sri Lanka erosion and chemical weathering rates are among the lowest for global mountain denudation. In this tropical humid setting highly weathered deep saprolite profiles have developed from high grade metamorphic charnockite during spheroidal weathering of the bedrock. The spheroidal weathering produces rounded corestones and spalled rindlets at the rock saprolite interface. I used detailed textural mineralogical chemical and electron microscopic (SEM FIB TEM) analyses to identify the factors limiting the rate of weathering front advance in the profile the sequence of weathering reactions and the underlying mechanisms. The first mineral attacked by weathering was found to be pyroxene initiated by in situ Fe oxidation followed by in situ biotite oxidation. Bulk dissolution of the primary minerals is best described with a dissolution – re precipitation process as no chemical gradients towards the mineral surface and sharp structural boundaries are observed at the nm scale. Only the local oxidation in pyroxene and biotite is better described with an ion by ion process. The first secondary phases are oxides and amorphous precipitates from which secondary minerals (mainly smectite and kaolinite) form. Only for biotite direct solid state transformation to kaolinite is likely. [...]