Abstract The objective of this study is to assess the effect of temperature and molecular weight of the soft segment on the degree of mixing between the hard and soft segments in segmented polyurethanes. The method of study consists of measuring electron spin resonance (e.s.r.) spectra of a nitroxide spin label in a series of segmented polyether polyurethanes (PU), in the temperature range 100–450 K. The PU are based on 4,4′-diphenylmethane diisocyanate (MDI), poly(tetramethylene oxide) glycol (PTMO) and 1,4-butane diol (BD) as the chain extender. The polymers were prepared from four molecular weights ( MW) of PTMO: 650, 1000, 2000 and 2900. The hard-segment content is constant in all polymers prepared (38 wt%). The nitroxide probe 4-hydroxy-2,2′,6,6′-piperidine-1-oxyl (TEMPOL) was attached to the polymer chains by reaction with an -NCO group of MDI. The nitroxide label is therefore located at a chain end. Two sites for the label, differing in their dynamical properties, were detected in all polymers around 300 K and most likely represent the hard and soft domains of the PU. Analysis of the e.s.r. spectra indicates that at ambient temperature (300 K) the increase in the MW of PTMO decreases the degree of mixing between the two segments in the PU. At 400 K the trend is reversed, and the PU containing the soft segment with the largest molecular weight has the highest degree of phase mixing. The lineshapes detected at 400 K can be interpreted in terms of a distribution of label sites, characterized by widths δg and δA of the g iso and A iso (from 14N) values, respectively, and reflecting the polarity profile in the polymers.