A comparison of two different models addressing the scalar transport in large-eddy simulations is conducted for a non-reacting jet and an experimental flame. A simple approach based on a gradient diffusion closure is compared against the linear-eddy model in the context of hydrogen-enriched non-reacting fuel jets and flames burning hydrogen-enriched mixtures. The results show that the gradient diffusion model is not valid as a subgrid scale model for large-eddy simulations of mixtures containing hydrogen. It produces unphysical scalar fields with unrealistic temperature distributions. Approaches based on the linear-eddy model can be used instead to obtain appropriate representation of the scalar field and more accurate predictions of the scalar transport and the temperature field.