Experimental methods are detailed for comprehensive further investigation of the turbulent premixed flames of natural-gas/air mixtures stabilized on a Bunsen-type burner already studied by joint imaging of OH and velocity (Frank et al., 1999). Simultaneous two-dimensional measurements of reaction progress variable and OH mole fraction are made from planar imaging of Rayleigh scattering and laser-induced fluorescence of OH. Image in-plane and out-of-plane spatial resolution of the order of 100 Am has been achieved. Care is taken to ensure that the instantaneous flame-front structure can be adequately resolved, and the measured scalar structure in laminar flames agrees well with flame calculations with a C-3 mechanism. Mean velocity and turbulence intensity profiles are presented for the non-reacting flows together with measurements of longitudinal and transverse correlation functions, their associated length scales, and the dissipation rate of the turbulence kinetic energy. In this paper. we report the mean structure of the turbulent flame brush in terms of Reynolds-and Favre-averaged profiles of the reaction progress variable and its standard deviation, and selected probability density functions. The results are compared with those derived from the OH images using the thin flamelet assumption. The mean and standard deviation of OH mole fraction are also presented. Preliminary conclusions are drawn about the relationship of the mean flame brush structure to the turbulence and the admixture of co-flowing air. The validity of the thin-flamelet assumption appears to be questionable for the lean flames investigated. Results for the structure of the instantaneous flame fronts are reported in a companion paper (Chen and Bilger, 2001).