Abstract A set of 22 biochars from different feedstock and pyrolysis conditions were produced using the same fixed bed pyrolysis reactor. Original substrates included softwood, hardwood and herbaceous biomass (pine, bark, cornstalk, miscanthus, poplar, switchgrass), microalgae (Desmodesmus communis, spirulina), wastes and residues (chicken manure, mushroom litter, olive pomace). Biochars were characterized by ultimate and proximate analysis and by analytical pyrolysis (Py-GC–MS). Parameters characteristics of the thermally labile fraction were obtained from thermogravimetric analysis (volatile matter, Tmax) and Py-GC–MS (molecular ratios). Volatile matter of biochars from a cornstalk thermosequence was strongly correlated with H/C ratios, while Tmax could be measured only for poorly carbonized biomass. Pyrolysis yields from Py-GC–MS were correlated with volatile matter. The molecular ratio toluene/naphthalene was governed by the extent of carbonisation and the presence of proteins in the original substrate. The 1-methylnaphthalene/naphthalene ratio was a general index of the thermal stability of biochar less influenced by the composition of the original feedstock. The indole/1-methylnaphthalene ratio was correlated with N/C ratio, while methylthiophene and benzothiophene were detected in the pyrolysate of sulphur-rich biochars from manure and litter. A coherent set of indices were obtained from TGA and Py-GC–MS for biochars with H/C > 0.3. In addition, Py-GC–MS provided information on the origin of biochar.