Kinking in natural and experimentally deformed biotite and muscovite has been investigated by transmission electron microscopy (TEM). It is found that complex microstructures exist in the vicinity of kink band boundaries (KBBs). Gentle bending of (001) planes plays an important role in the kinking process; where (001) planes are bent several degrees, edge dislocations are observed in complex walls between active slip planes together with broad bands of dislocations parallel to (001). Microfracturing and dilation occur parallel to (001) planes and the KBB. A model is presented to explain the origin of observed KBBs which may be applicable to other crystalline materials with only one active slip plane.