In order to investigate the effect of potassium addition on the composition of torrefaction condensates, two demineralized wood species were impregnated with different concentrations of K2CO3 and then torrefied at 275 °C up to an anhydrous weight loss (AWL) of 25%. Torrefaction was carried out in both a thermogravimetric analysis (TGA) instrument and a laboratory fixed-bed reactor. Condensates from the fixed bed reactor were collected and analyzed by Gas Chromatography-Mass Spectroscopy (GC–MS). TGA of raw and K2CO3-impregnated biopolymers (cellulose, xylan and lignin) were performed to facilitate interpretation of the results. TGA showed that when potassium content increased in the biomass, shorter torrefaction times were sufficient to obtain the targeted AWL. GC–MS showed, for both wood species, that potassium promotes the formation of acetol and slightly enhances acetic acid yield. The amount of some lignin derivatives (guaiacol, syringol, 4-vinylguaiacol) also rose with potassium addition. Yields of levoglucosan, LAC (1-hydroxy-(1R)-3,6-dioxabicyclo[3.2.1]octan-2-one) and DGP (1,4:3,6-dianhydro-a-d-glucopyranose), as well as furfural and 5-hydroxymethylfurfural, decreased drastically in the presence of potassium. In conclusion, small additions of potassium carbonate deeply affected thermal degradation of wood species and the speciation of torrefaction condensates.