Cryosols from Maritime Antarctica have been less studied than soils from continental areas of Antarctica. In 2003, an extensive soil survey was initiated at King George Island within the scope of the Brazilian Antarctic Program. In the present work XRD, DXRD, DTA/TG, TEM/EDS and selective chemical dissolution were used to characterize the clay fraction of basaltic, acid sulphate and ornithogenic cryosols from ice-free areas of Admiralty Bay, King George Island. Randomly interstratified smectite:hydroxy-Al interlayered smectite is the main clay mineral of basaltic soils. For these soils, the presence of clay-sized pyroxene and feldspars indicate the low degree of chemical alteration of the parent material. Nevertheless, hydroxyl-Al interlayering of inherited smectite is likely to be pedogenic. Also, the detection of about 14 % of a Si-rich allophane-like phase in the clay fraction suggests that dissolution of primary alumino-silicates is a major process, with limited leaching of dissolved Al-Si. Kaolinite, chlorite and regularly interstratified illite-smectite predominate in acid sulphate soils, in which jarosite is also an important component of the clay fraction. Although feldspar is abundant in the sand fraction, clay-sized feldspar was not present suggesting that acid chemical weathering is an important process at these sites and may account for the formation of kaolinite. Crystalline Al and Fe phosphates occur only at sites directly affected by penguin activity and constitute up to 30 % of the clay fraction. Highly reactive non-crystalline Al, Si, Fe and P phases control the chemical characteristics of ornithogenic sites, accounting for over 50 % of the clay fraction for some sites. Contrary to Antarctic continental cryosols, where physical weathering dominates, chemical weathering is an active process in cryosols from Maritime Antarctica and is enhanced by the oxidation of sulphides present in the parent material and at some locations is affected by faunal activity.