We present the first results on optical properties (λ ∼ 540 nm) of fresh aerosols from the combustion of Indonesian peat, German peat and other types of biomass, measured under controlled laboratory conditions. The mass scattering and mass absorption efficiencies for Indonesian and German peat aerosols are in the range of 6.0–8.1 and 0.04–0.06 m² g⁻¹, respectively. A very high single scattering albedo (0.99) is observed for the peat smoke aerosols, reflecting the smoldering burning conditions (emission ratio, ΔCO/ΔCO₂ = 19–50%). The relative increase in light scattering (f(RH)) due to an increase in relative humidity (RH) from 15% to 90% is very low (i.e., f(90) = 1.05) for both Indonesian and German peat aerosols. This value is considerably smaller than for aged Indonesian peat smoke particles (f(80) = 1.65) [Gras et al., 1999]. This suggests that atmospheric aging processes may be an important factor for aerosol hygroscopicity.