As part of our continuing effort to understand microphase separation of poly(urethane urea) block copolymers, FTIR spectroscopy and thermal techniques (DSC and DMA) were used to investigate the phase behavior of two series of MDI-polytetramethylene oxide soft segment copolymers, chain-extended with ethylene diamine or a diamine mixture. Due to the complex nature and multiple absorbances in the carbonyl and N-H regions of the FTIR spectra, quantitative analysis was not possible. However, qualitative trends could be discerned, and the spectral changes were found to be in excellent agreement with our previous quantitative analysis of the same copolymers using small-angle X-ray scattering. DSC and DMA experiments both indicate that the soft phase T-g decreases with increasing hard segment content. This is contrary to increased hard segment mixing in the soft phase, but can be rationalized by taking into consideration soft segment crystallinity and the concentration of 'lone' MDI units in the soft phase. (C) 2003 Elsevier Science Ltd. All rights reserved.