Polyacetaldehyde has been prepared in the liquid and vapor phases without the addition of a catalyst by storing undiluted acetaldehyde in the dark at a temperature of −68°C. for 3–6 months. When acetaldehyde was stored in an untreated Pyrex flask, 20% high polymer, 55% paraldehyde, and less than 1% metaldehyde were obtained. The polymer was an elastomeric solid with a low to moderate degree of crystallinity and an intrinsic viscosity of 0.63 dl./g. When the sample was stored in a Pyrex flask which had been pretreated with alcoholic potassium hydroxide followed by a water wash, 16–17% high polymer and 6% paraldehyde were obtained. Some polymer formed on the unwetted glass above the liquid. It was slightly elastic, at least moderately crystalline, and had an intrinsic viscosity of 1.6 dl./g. It is suggested that the formation of this polymer was initiated by basic active sites on the Pyrex surface which acted by an anionic mechanism. Also some polymer was formed in the liquid phase. This material was an amorphous, highly viscous oil with an intrinsic viscosity of 0.28 dl./g. It was probably formed by a cationic mechanism. It is also suggested that a relatively small amount of paraldehyde was formed in the alkali-treated flask in the liquid phase due to neutralization of the Brønsted acid sites and possibly some of the Lewis acid sites on the surface of the flask.