High-molecular-weight copolymers of epsilon-caprolactone (CL) and 1,5-dioxepan-2-one (DXO) over the complete range of compositions have been synthesized and studied. Stannous 2-ethylhexanoate was used as a catalyst and the polymerization was carried out in bulk. Reactivity ratios were r(CL) = 0.6 and r(DXO) = 1.6, which give r(1)r(2) = 1, thus leading to an ideal copolymer. The copolymers exhibited crystallinity as long as the proportion of 1,5-dioxepan-2-one did not exceed 40%. The crystallinity and melting temperature of the copolymers decreased with increasing content of 1,5-dioxepan-2-one. Analysis of the melting-point depression data of the copolymers indicates that about 40% of the 1,5-dioxepan-2-one comonomer units are incorporated into the CL crystals.