We report experimental excitation and characterization of surface plasmon modes in planar metal–insulator–metal (MIM) waveguides. Our approach is based on determining the width of the reflection angular spectrum in the attenuated total reflection (ATR) configuration. Owing to its transverse character, the ATR configuration provides a more straightforward and simpler way to determine the loss of plasmonic modes in MIM structures, compared to using tapered end couplers with multiple waveguide samples or scanning near-field optical microscopy. In this Letter, two waveguide structures with Au claddings and $50/200\mathrm{nm}$ ${\mathrm{SiO}}_2$ cores are investigated. The propagation lengths measured at $\lambda =1.55\mathrm{\mu m}$ are 5.7 and $18\mathrm{\mu m}$ , respectively, in agreement with the theoretical predictions.