Gold nanodecahedra (pentagonal bipyramids) exhibit striking optical properties spanning the visible and near-IR spectral regions, which together with a high electric-field enhancement at their tips, makes them very promising materials for sensing-related applications. The effective design of biosensors based on these nanocrystals requires a detailed investigation, and understanding of their surface plasmon resonances. For that purpose, we have used correlated electron microscopy imaging and optical dark-field spectroscopy on individual decahedra to investigate their size and orientation-de pendent optical properties at the single particle level. We have also investigated the effects-of tip truncation and local refractive index changes on single particle plasmon resonances. The experimental results are supported by theoretical modeling based on a fully 3D, boundary element method (BEM-3D).