Laser-assisted electrochemical size-reduction has been carried out on silicon nanostructures produced by electron beam lithography and reactive ion etching. We demonstrate the ability to reduce nanopillars down to 10 nm diameter while preserving shape, but also the possibility of preferential etching of different parts of the pillar by varying the applied bias voltage. Furthermore, the origin of the carriers responsible for the etching is discussed, and we note the presence of a ‘dark’ etching mechanism working in parallel with the normal dissolution reaction. Finally, the etching of shallow Si dots on a Si surface shows further localization of etching, with a different etching reaction taking place in the vicinity of the structures as opposed to the planar surface, far from the structures.