Characteristics of smoke particulates generated from a wood fire in a ventilated model tunnel were investigated using an in situ optical and a grid sampling technique. Volume-to-surface mean diameter and mass concentration of the smoke particles, and the transmission, optical density per unit length and particulate optical density of the smokeladen exhaust gas were obtained as a function of the burning process in the tunnel. It was found that high concentrations of smoke (-1 mg I-]) were rapidly generated as the fire changed from oxygen-rich to fuel-rich burning, resulting in fast obscuration of the passageway. The simultaneous generation of large amounts of smoke and high temperature carbon monoxide ( -8 %) coupled with the low transmission (-1 % through 0.5 m) represents an extremely hazardous situation in such a fire environment. Present measurements and others from current smoke testing chambers are compared and discussed.