Copolymers of 3,3-bis-hydroxymethyloxetane, BHMO, 3-methyl-3-hydroxymethyloxetane, MHMO, or with 3-ethyl-3-hydroxymethyloxetane, EHMO, monomer units were characterized by x-ray fiber diffraction, differential scanning calorimetry and C-13 solid-state nuclear magnetic resonance (NMR). The copolymers are statistically random and crystalline throughout the range of compositions. Both P (BHMO) and P(MHMO) appear to crystallize in the same crystal form. The fiber repeat indicates a planar zigzag backbone conformation, c(fiber axis) = 4.77 +/- 0.03 angstrom. Similarities in the x-ray fiber diagrams as well as a linear dependence of T(m) with composition of copolymer with no change in fiber diagrams indicates isomorphism, a phenomenon in which the random substitution of MHMO monomeric units into the crystalline lattice of P(BHMO) occurs without hindering crystallization of the resulting copolymer.