The changes in phase structure under mech. deformation of poly(butylene terephthalate) (PBT)/poly(tetramethylene oxide) (PTMO) multiblock copolymers differing in the amt. and block length of PTMO were investigated by 13C magic-angle-spinning NMR. Measurements were performed on unstretched samples, on stretched samples allowed to relax before the NMR expt., 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 obsd. (TCH, 13C-T1r, 1H-T1r), which is attributed to a microphase sepn. 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 addnl. resonance with different chem. shifts for the OCH2 carbons of PTMO and with a restricted mobility was obsd. This new resonance, which is also found in unstretched samples at -30 Deg, is assigned to strain-induced cryst. PTMO. The amt. of cryst. PTMO increases linearly with the sample strain. It appears that, in stretched samples, heating to 50 Deg leads to irreversible melting of the PTMO crystals, in contrary to the in-situ stretched samples, which show recrystn. upon cooling to room temp. 2D rotor-synchronized 13C-CPMAS expts. revealed a high orientation of the hard and soft phases upon stretching.