Recent studies of subgrid-scale (SGS) mixing and turbulence-chemistry interaction have shown that turbulent flames contain different structures. In flamelet structures diffusion of reactive scalars and chemical reaction are tightly coupled. Most mixing models used in probability density and filtered density methods, however, are based on non-reactive scalars. To investigate the effects of the coupling on the diffusion we decompose a reactive scalar into a steady flamelet part and perturbations from it. The diffusion of the former can be obtained from a suitably chosen flamelet solution while the latter is unclosed. The conditionally filtered diffusion and dissipation of the reactive scalar perturbations are analyzed using high-resolution line images obtained in turbulent partially premixed (Sandia) flames. For SGS scalar containing flamelets, the perturbation diffusion has characteristics similar to that of a non-reactive scalar, in contrast with the flamelet part. The functional form of the conditionally filtered diffusion is well described by the Interaction by Exchange with the Mean (IEM) model. Our perturbation analysis of the flamelet equation shows that for perturbations having length scales smaller than the reaction zone width, the reactive scalar diffusion is largely controlled by the mixture fraction field, thus having the characteristics of non-reactive scalar mixing. For perturbations with length scales larger than the reaction width, the conditionally filtered diffusion has the same form as non-reactive scalar mixing, with the mixing time scale determined by the flamelet. The IEM model predictions based on this mixing time scale are in good agreement with the experimental results for a range of SGS conditions, suggesting that the perturbations are consistent with unsteady flamelets for the conditions studied. Thus, mixing models based on non-reactive scalars can potentially model the SGS mixing accurately. The results in the present study can be useful for developing a unified mixing model that can predict all combustion regimes accurately. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.