Tunable diode laser absorption spectroscopy (TDLAS) was employed to measure several important process parameters at two different locations inside the reactor of an atmospheric air-blown 0.1 MW biomass gasifier. Direct TDLAS at 2298 nm was employed for CO and water calibration-free scanned wavelength modulation spectroscopy at 1398 nm for H2O and gas temperature and direct TDLAS at 770 nm for gaseous elemental potassium K(g) under optically thick conditions which correspond the first in situ measurements of K(g) and temperature in a reactor core and in biomass gasification respectively. Actual average temperatures in the reactor core were significantly higher than the uncorrected thermocouple measurements in the gas stream. The CO concentrations at the lower optical access port were comparable to those obtained by GC at the exhaust. In gasification mode similar H2O values were obtained by the two different TDLAS instruments. The reaction time was faster for peat than for stem wood.