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The properties of the leaky surface plasmon polariton (SPP) modes in gold nanostripes were investigated using scattered light microscopy. Both bare gold nanostripes and stripes coated with a thin polymer film containing a near-infrared absorbing dye were examined. Real-space microscopy images were employed to determine the SPP propagation length, while Fourier space images provided measurements of the wavevector. Frequency versus wavevector dispersion curves were generated by performing experiments at different excitation wavelengths, and the slopes of these curves yielded the SPP group velocities. For the bare nanostripes the group velocity was determined to be vg = (0.92 ± 0.05)c0 and for the dye-coated nanostripes it was vg = (0.85 ± 0.06)c0, where c0 is the speed of light. The SPP lifetimes were estimated by combining the group velocity and propagation length measurements. The results show that the lifetime of the gold SPPs is significantly reduced when the nanostripes are coated with the dye. At the peak of the dye absorption curve the change in the SPP dephasing rate induced by the dye–polymer film was found to be 0.07 fs–1. Finite element simulations show that the increased dephasing is due to a combination of energy transfer from the SPP modes to the dye, as well as increased radiation damping due to changes in the dielectric environment of the nanostructures. These findings provide insights into the energy transfer processes in plasmonic systems, which can be leveraged to optimize the design of plasmonic devices for applications in sensing, imaging and nanophotonic circuits.