The thermal degradation of poly(epsilon -caprolactam) and its copolymers with epsilon -caprolactone has been studied by high-resolution thermogravimetric analysis (TGA), followed by the analysis of evolved gas via a simultaneous coupling to Fourier transform infrared spectrometry (FTIR) and mass spectrometry (MS). Poly(E-caprolactam) and poly(epsilon -caprolactone) polymers are, respectively, degraded at different temperatures, rendering this study to be particularly useful in determining the structure of their copolymers, i.e. random or blocky-like copolymers. The poly[(epsilon -caprolactam)-co-(epsilon -caprolactone)] polymers have been synthesized in bulk by anionic ring-opening copolymerization promoted by epsilon -caprolactammagnesium bromide in a temperature range from 120 to 180 degreesC. A multi-step degradation was observed by TGA, and characterization of the evolved gas by FTIR and MS gives credit to some blocky-like enchainment of the comonomers, This assignation has been further confirmed by TGA directly performed on blends of poly(epsilon -caprolactam) and poly(epsilon -caprolactone) as obtained by solvent casting.