The thermal degrdn. behavior of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-co-HHx), HHx = 12 mol%) has been studied under different environmental conditions by thermogravimetric anal. (TGA) Fourier transform IR (FT-IR) spectroscopy. It is reported that at higher temp. (>400 DegC) carbon dioxide and propene are formed from the decompn. product crotonic acid in a nitrogen atm., whereas in an oxygen atm. propene oxidizes in a further step to carbon dioxide, carbon monoxide and hydrogen. It was also found that PHB and P(HB-co-HHx) have a similar thermal degrdn. mechanism. The anal. of the FT-IR-spectroscopic data was performed with 2D and perturbation-correlation moving-window 2D (PCMW2D) correlation spectroscopy.