Reactive pyrolysis of copolymer type polyacetals in the presence of cobalt sulfate incorporated into pyrolysis-gas chromatography equipped with a high-resolution capillary separation column was applied to the study of sequence distributions. The ethylene oxide (E) contents and the distributions of E sequences up to E(7) in the polymer chain were evaluated on the basis of peak intensities of cyclic ethers containing E units on the pyrogram. These vlaues were in good agreement with those obtained by hydrolysis.