The effect of hydrogen bonding on the phase behavior of a chemically similar polyurethane and polyurea and their blends with poly(ethylene glycol) is examined. The polyurethane and polyurea were synthesized from the same diisocyanate, 1,5-diisocyanato-2-methylpentane, using an aromatic diol and aromatic diamine, respectively. Fourier transform infrared spectroscopy was used to characterize the distribution of hydrogen bonds in these polymers and their blends. The distribution of hydrogen bonds in the polyurethane homopolymer was found to be quite similar to that found in an amorphous polyurethane studied previously in this laboratory. However, upon annealing, some sort of ordered structure was detected spectroscopically. The polyurea formed an equivalent ordered structure much more readily at room temperature. Ordered hydrogen-bonded domains were also detected in the spectra of the blends, either after an extended time at room temperature or after annealing at an elevated temperature. The formation of ordered structures occurred at temperatures well below the thermally measured glass transition. Melting endotherms could not be detected in most of these samples. It is suggested that the order present in these blends might be largely two-dimensional. The hydrogen bonds between adjacent urethane (or urea) units can align to form a sheet, with the methyl group that is part of each segment essentially laying in a plane that is perpendicular to this structure. Because the methyl group is asymmetrically placed in the diisocyanate used to synthesize these polymers, packing between sheets would be imperfect at best.