Stable, high molecular weight polymers of formaldehyde have been studied to determine their molecular structure. The polymer chain was shown to be a polyoxymethylene by chemical analysis and infrared absorption spectra. Infrared techniques also revealed hydroxyl end groups which were replaced by acetate groups on esterification. The endgroups, which were also measured by chemical methods, were used to determine the number‐average molecular weight over the range 20,000 to 98,000. Values obtained by assuming two endgroups per molecule agreed with osmotic pressure results, thus showing this to be an essentially unbranched polymer. Two theoretically possible branch‐point structures were investigated by chemical methods; none could be detected, further indicating a linear polymer structure. Indirect evidence of narrow molecular weight distribution was obtained by comparing melt viscosity properties with those of other polymers of known wide and narrow distribution.