The solid-state organization of copolymers based on methylene oxide (MO) units and tetramethylene oxide (T) units, with T unit contents ranging between 0.9 and 10.0 mol %, is investigated by the combined use of solid-state NMR, WAXS, and DSC experiments. In these semicrystalline copolymers, the T units can be viewed as linear defects inserted along linear poly(oxymethylene) chains. As expected, the insertion of T units induces a significant decrease of both crystallinity and crystallite size: a large part of T units is located in the amorphous domains. However, some T units can also be detected within the crystalline domains and/or the interfacial regions with the amorphous phase. More precisely, the amount of T units at both sides of the noncrystalline/crystalline interfaces seems to be much higher than the T units in the interior of the crystallites. At the lowest T unit content (0.9 mol %), the composition averaged over both interfacial and crystalline phases appears to be identical to the composition of the amorphous phase. When the comonomer content increases, the amount of T units in the interfacial and crystalline zones becomes higher and higher while the partitioning coefficient of the T units in these domains tends to a limiting value of 0.40.