The flames of forest fuels form a weakly ionized gas. Assuming a Maxwellian velocity distribution of flame particle and collision frequencies much higher than plasma frequencies, the propagation of microwaves through forest fuel flames is predicted to have attenuation and phase shift. A controlled fire burner was constructred where various natural vegetation materials could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality neutwork analyze to measure phase and attenuation. The controlled fires had temperatures in the range of 500-1000 K and microwaves attenuation of 1.0-4.5 dB m(-1) was observed across the 0.5 m diameter cavity. Attenuations of this magnitude could affect active remote sensing systems signals at microwave frequencies in forest fire environments where flame depths of up to 50 m are possible. In the experiment, temperature was not the only controlling parameter for the ionisation; type of fuel burnt also influenced it. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analysis of the composition of the fuel confirmed that a higher content of alkali (with low ionization potential) lead to higher electron densities. Electron densities in the range of 0.32-3.21 x 10(16) m(-3) and collision frequencies of 1.1-4.3 x 10(10) s(-1) were observed for flames with temperature in the range of 730-1000 K. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.