Flammability (studied in terms of limited O-2 index (LOI) and thermovision) of modified segmented polyurethanes prepared by addition of partially polymeric diphenyl methane 4,4'-diisocyanate (MDI, 31% of free isocyanate groups per pure MDI) and 1.6-hexamethylene diisocyanate (HDI) to polyoxopropylenediol (M =750) with 1,2-propanediol and 3-chloro-1,2-propanediot used as low-M chain extenders, was studied in relation to 3-chloro-1,2-propanediol percentage. At 40016, LOI was higher by similar to 1.5%. With up to 15% of MDI replaced by HDI, LOI remained unaffected. At more Cl built into rigid segments, free combustion was more vigorous and irregular, whereas LOI combustion was stable and combustion-zone temperatures were lower in either mode.