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The Denchai gem sapphire deposits in Phrae Province, northern Thailand are closely associated with late Cenozoic alkaline basaltic rocks. The sapphires occur in alluvial placer deposits in palaeo-channels at shallow depths. Electron microprobe analysis of minor and trace element contents (Fe, Ti, Cr, Ga and V) of the sapphires indicate the following oxide abundances: Fe 2 O 3 (0.32À1.98 wt.%), TiO 2 (0.01À0.23 wt.%), Cr 2 O 3 (<0.01 wt.%), Ga 2 O 3 (0.01À0.03 wt.%) and V 2 O 5 (<0.03 wt.%). Optical studies of sapphires revealed three types of primary fluid/melt inclusions. CO 2 -rich inclusions (Type I) contain three phases (LH 2 O + LCO 2 + V) with the vapour phase comprising <10À15 vol.%. The presence of CO 2 was confirmed by microthermometry and laser Raman analysis. Polyphase inclusions (Type II) (vapour + liquid + solid) contain a fluid bubble (20À30 vol.%), an aqueous phase (10À15 vol.%) and several solid phases. Silicate-melt inclusions (Type III) comprise vapour bubbles, silicate glass and solid phases. Proton-induced X-ray emission (PIXE) analysis revealed high concentrations of K (~4 wt.%) as well as Ca (~0.5 wt.%), Ti (~1 wt.%), Fe (~2 wt.%), Mn (~0.1 wt.%), V (<0.03 wt.%), Rb (~70 ppm) and Zr (~200 ppm) in the silicate glass. The Ga 2 O 3 abundances and Cr 2 O 3 /Ga 2 O 3 values (<1) of the sapphires favour their formation by magmatic processes. The presence of CO 2 -rich fluids and high K concentrations in the silicate melt inclusions link the origin of the Denchai gem sapphires to CO 2 -rich alkaline magmatism.