Minimum potential energy helical conformations for a family of four isotactic polyacetaldehydes have been determined. Our results indicate that all of the polymers form irrational helices. Comparisons have been made with the reported structures for two of these stereoregular polymers based on earlier X-ray diffraction data, c-Axis values associated with the pitch of the helix for polyacetaldehyde and for polytrichloroacetaldehyde (polychloral) were experimentally measured to be 0.48 and 0.51 nm, respectively. Our calculated conformations afforded values for a helix pitch of 0.47 and 0.52 nm, respectively, which derive from a 3.9/1 helix for polyacetaldehyde and a 3.7/1 helix for polychloral. The structure for polytribromoacetaldehyde (polybromal) was predicted to be similar to that for polychloral. For polytrifluoroacetaldehyde (polyfluoral) and polyacetaldehyde, a number of helical conformations with similar energies were found. All of these conformations could be related to the polychloral helical structure.