The upward propagation of a lean methane-air flame in a vertical tube is investigated. The shape and the velocity of the flame front are extracted from video records, and the velocity of the flow induced by the flame is measured with particle image velocimetry (PIV) using alumina particles to seed the gas. The thermophoretic drift of these particles relative to the gas is shown to cause an error in the PIV measurements in the transport region of the flame, where the temperature gradient is large. An iterative correction of the measured velocity is proposed that uses simplified, quasiunidimensional energy and species conservation equations to compute an approximation to the profile of gas temperature across the flame in terms of the profile of gas velocity. The correction is tested using synthetic velocity fields and applied along the axis of the tube. The results of the quasiuni-dimensional model also show that a conduction-radiation balance is approached in a region of low velocity that develops behind the flame front when the flammability limit is approached. Radiation losses from the combustion products in this region then become important and may cause the extinction of the flame.