The isothermal crystallization of a semicrystalline poly(ethylene oxide-b-ethylene/butylene) diblock copolymer (PEO-b-PBh) in uniform lamellar films has been investigated by X-ray reflectivity, optical microscopy, and atomic force microscopy. Crystallization of the PEO block leads to an increase in the lamellar thickness of both blocks. As the density of PEO increases upon crystallization, this effect is accompanied by a contraction in the lateral direction, which results in cracking of the film. The combination of the different techniques allows construction of a complete model with an integer or half-integer number of folds in the vertically oriented crystalline stems.