The changes in phase structure under mechanical deformation of poly(butylene terephthalate) (PBT)/poly(tetramethylene oxid) (PTMO) multiblock copolymers differing in the amount and block length of PTMO have been investigated by C-13 magic-angle-spinning NMR. Measurements have been performed on unstretched samples, on stretched samples allowed to relax before the NMR experiment, and on samples that are kept under tension in the spinning rotor (in-situ stretched). For unstretched samples a heterogeneity in NMR relaxation behavior of the OCH2 carbons of PTMO was observed (T-CH, C-13-T-1 rho, H-1-T-1 rho), which is attributed to a microphase separation in the amorphous phase into a PTMO-rich phase (mobile) and a mixed PBT/PTMO-phase (restricted mobility). Long PTMO block lengths and high PTMO contents favor the formation of the PTMO-rich phase. For stretched and in-situ stretched samples with relatively long PTMO block lengths, an additional resonance with different chemical shifts for the OCH2 carbons of PTMO and with a restricted mobility was observed. This new resonance, which is also found in unstretched samples at temperatures of -30 degrees C, is assigned to strain-induced crystalline PTMO. The amount-of crystalline PTMO increases linearly with the sample strain. It appears that, in stretched samples, heating to 50 degrees C leads to irreversible melting of the PTMO crystals, in contrary to the in-situ stretched samples, which show recrystallization upon cooling to room temperature. 2D rotor-synchronized C-13-CPMAS experiments revealed a high orientation of the hard and soft phases upon stretching.