We present an experimental study of the growth rate of the Darrieus-Landau instability on propane-air and on oxygen enriched propane-air flames using a two-dimensional inverted-"V" flame on a slot burner. We use a novel system of electrostatic deflection to produce an initial perturbation of controlled wavelength and amplitude. The growth rate of this spatially growing perturbation is measured as a function of wavenumber, of equivalence ratio and of oxygen enrichment using high speed flame imaging. The results are analyzed using the Clavin-Garcia laminar flame stability theory (including temperature dependent diffusivities). The dependence of the growth rate on wavenumber and flame speed is reasonably well represented by this theory. As a by-product of this analysis, we obtain values of the Markstein number, which, for rich flames, are found to decrease with equivalence ratio and to increase with oxygen concentration.