Spin-probe research into nanometer-scale molecular motions occurring in oil-extended styrene-butadiene-styrene diblock copolymers within the glass-transition range is reported. Complex resonance spectra are deconvoluted, and their intricate temperature dependence, resulting from the convolution of individual spectra due to nitroxide molecules located in different phases of the block copolymers, is analyzed. It is proved that a Boltzmann sigmoid accurately describes the temperature dependence of outer line separation. The characteristic inflection point of each sigmoid is used to assign an inflection temperature to each phase of the block copolymers. The inflection temperature coincides with the narrowing temperatures, within the experimental error. It is concluded that the inflection temperature is estimated with a higher accuracy than the narrowing temperature and that it allows a more profound analysis of molecular motions. The anomalous dependence of the glass, narrowing, and inflection temperatures for spin probes immobilized within the hard phase on the oil content has been assigned to morphological modifications, on the nanometer scale, induced by oil molecules added to the block copolymers. The experimental data demonstrate the sensitivity of spin-probe data to modifications of the nanometer-size architecture of block copolymers. (C) 2004 Wiley Periodicals, Inc.