Simultaneous imaging of depolarized and polarized Rayleigh scattering combined with OH-LIF and two-photon CO-LIF provides two-dimensional measurements of mixture fraction, temperature, scalar dissipation rate, and the forward reaction rate of the reaction CO + OH = CO2 + H in turbulent partially premixed flames. The concept of the three-scalar technique, for determining the mixture fraction using CO-LIF with depolarized and polarized Rayleigh signals was previously demonstrated in a partially premixed CH4/air jet flame [J.H. Frank, S.A. Kaiser, M.B. Long. Proc. Combust. Inst. 29 (2002) 2687-2694]. In the experiments presented here, we consider a similar jet flame with a fuel-stream mixture that is better suited for the diagnostic technique. The contrast between the depolarized and the polarized Rayleigh signals in the fuel and air streams is improved by partially premixing with an argon/oxygen mixture that has the same oxygen content as air. The substitution of argon, which has a zero depolarization ratio. for the nitrogen in air decreases the depolarized. Rayleigh signal in the fuel stream and thereby increases the contrast between the depolarized and the polarized Rayleigh signals. We present a collection of instantaneous 2-D measurements and examine conditional means of temperature, scalar dissipation, and reaction rates for two downstream locations. The emphasis is on the determination of the scalar dissipation rate from the mixture-fraction images. The axial and radial contributions to scalar dissipation are measured. The effects of noise on the scalar dissipation measurements are determined in a laminar flame, and a method for subtracting the noise contribution to the scalar dissipation rates is demonstrated. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.