The etching behaviour of the double-layered (001) faces of potassium bichromate crystals was studied by using in situ atomic force microscopy (AFM). These crystals were cleaved in air or ethanol along (001), and the opposite cleavage faces (001) and (001) were then etched in ethanol or a water/ethanol mixture. We found that in air an etch resistant layer is formed. Whether this layer can be removed depends on the exposure time to air, on the water content of the etch mixture and on the amount of scanning by the AFM. Under specific conditions this allows us to do lithography on the surface. On the air-cleaved faces, only at some positions expanding etch pits developed, in which layer-by-layer etching took place at freshly etched parts on the flat bottom. Most etch steps were of height d(001) as expected for the double-layered structure of K-2 Cr-2 O-7. However, sometimes split steps of half height d(002) were encountered. Both the shapes of the 2D nucleation etch pits and the molecular-resolution images were similar on the (001) and the (001) faces, which suggests PT symmetry. Therefore on this molecular scale, the hypomorphism (P1 symmetry in this case) visible on macroscopic K2Cr2O7 crystals appears to be absent. From the orientation of the d(002) steps inside the etch pits the stable and the unstable (001) half layers A and B could be identified with respect to the crystal structure. During etching, A was nearly always on top of the crystal surface, proving that this is the most stable layer. (C) 2000 Elsevier Science B.V. All rights reserved.