Moisture cured polyurethane-urea (MCPU) is one of the industrially important polymers that shows good thermal, mechanical and weathering properties and is widely used in the reactive hot melt adhesives and coatings. Structural variation of the building blocks, i.e. soft, hard segment and chain extender structure on the phase mixing characteristics during cure in polyether based moisture-cured polyurethanes (MCPUs) has been investigated. Variations in the soft segment structure like polyethylene glycol (PEG), polypropylene glycol (PPG) and polytetramethylene glycol (PTMG) and hard segment like toluene diisocyanate (TDI) or isophorone diisocyanate (IPDI) were made. The effect of isocyanate content (NCO:OH ratio 1.6:1 and 2:1) as well as various aliphatic diol and aromatic diamine chain extenders were also compared. The phase mixing phenomenon during network growth was evaluated by differential scanning calorimetry (DSC) and a correlation was drawn for the degree of cure with the phase mixing property. The change in modulus and thermal stability with the cure advancement were measured by dynamic mechanical and thermal analysis (DMTA) and thermogravimetric analysis (TGA). A correlation was made for the soft, hard segment and chain extender structure to the phase mixing phenomenon during cure. The rate of phase mixing was found to be dependent on the subtle variations in molecular architecture. (c) 2006 Elsevier Ltd. All rights reserved.