The effect of fuel mass flux on the flow field of a I-m-diameter methane fire is examined using particle image velocimetry (PIV) and data from visible flame emission and polycyclic aromatic hydrocarbon (PAH) fluorescence. The fuel mass flux is varied from 0.040 to 0.066 kg/m(2)s, which is representative of a significant fraction of the range of fuel mass fluxes that occur in liquid hydrocarbon pool fires. The results show that increasing the fuel mass flux increases the size of the region of unmixed fuel vapor but has little effect on the dynamics in the mixing region within the first diameter above the burner surface. In-plane time-averaged velocities ((U) over bar, (V) over bar) and time-averaged turbulent stresses ((u'(2)) over bar, (v'(2)) over bar, and u (u'v') over bar) are given for a vertical plane passing through the burner centerline. Visible emission data are used to infer the probability of the maximum mixture fraction along a path normal to the PIN plane failing to exceed a stoichiometric value. PAH fluorescence data are used to infer the radial position of maximum time-averaged turbulent reaction rate as a function of elevation. Methane (0.054 kg/m(2) S) and hydrogen (0.022 kg/m(2)s) fires with the same inlet enthalpy flow rate (heat release rate) of 2.1 MW are examined. The comparison of results highlights the effect on the flow field of differences in fuel thermochemistry and density. The hydrogen fire reaches a maximum velocity at a lower elevation than that of the methane fire and has a correspondingly shorter unmixed fuel vapor region and greater lateral (radial) extent of the mixing region for a given elevation. The experiments were conducted with controlled and characterized boundary conditions for the purpose of providing validation data sets relevant to conditions found in fully turbulent pool fires. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.