An aluminosilicate ZSM-12 may be prepared de novo in a small crystalline form from a reaction mixture containing a source of silica and a source of alumina. A small crystalline form of aluminosilicate ZSM-12 may also be prepared from a small crystalline form of borosilicate ZSM-12 by replacement of boron in the borosilicate ZSM-12 framework with aluminum. The aluminosilicate ZSM-12 is useful as an isomerization selective catalyst in processes such as isomerization dewaxing hydrocarbon feedstocks.