The natural dimensional stability of wood is a function of its total water content. The success of a wood treatment depends on the extent to which the consolidating material replaces the water and where the replacement occurs in the wood structure. Polyethylene glycol (PEG) could be expected either to bulk the cell lumina or to infiltrate the cell walls, or do both. The final location of the PEG depends on its physical properties, and these depend on molecular weight. Previously, locating PEG within the cell wall layers was not possible. A new, simple stain has been developed for the light microscopical differentiation of PEG in wood. It is based on cobalt thiocyanate. This paper concerns its use to determine the location of PEG400, 3350, and 540 Blend in transverse sections of treated wood. (See also AATA 21-410)