Differential diffusion effects in a low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame are shown to lead to changes in the elemental mass fractions. The effect is caused by unequal diffusion of reactants away from the burner and products towards the burner. Analysis of data previously collected from this flame shows the local equivalence ratio to vary by as much as 25% from its inlet value, with similar variations found for the atomic C/O ratio (up to 25%) and atomic C/H ratio (up to 10%). Variations of comparable magnitude are predicted by detailed kinetic modeling of the same flame using a published mechanism. The variations predicted for a similar flame but at atmospheric pressure are slightly smaller than those at low-pressure, which is consistent with the constitutive equations. Similar effects would be expected for any flame in which the mass average flame velocity is comparable to the diffusion velocities of the major flame species. The results have implications for consideration of molecular-weight growth and soot formation in such systems, since these processes are strongly dependent upon the C/O and equivalence ratios.