Two polymer fibre types (PP and PAN) and different microstructural methods were used to compare the porosity of fibre mortar composites and the effect of rapid thermal exposure on the fibre structure. Polished thin sections were prepared from fibreless reference mortar and fibre mortar samples before and after fire impact. Polarizing light microscopy indicates increased air void porosity in the fibre mortars. Polarizing light microscopy and scanning electron microscopy reveal that PP and PAN fibres react entirely differently to heat. PP fibres melt, whereas PAN fibres become dark in appearance, presumably due to chemical transformations, and exhibit a zoned structure at the outer part of the fibre. A transition zone program shows that porosity detected by backscattered electron images is higher at the fibre/matrix contact zone than in the bulk matrix. Fire impact broadens this porosity region at the contact zone. On the whole, the porosity level at the contact zone seems to be higher in the PAN than in the PP fibre mortar.