The optical properties of a family of alumina membranes that contain Au nanoparticles with diameters of 52 nm and aspect ratios (length divided by diameter) varying from 0.38 (oblate or pancake-like) to 24 (prolate or needle-like) have been studied. The dynamic Maxwell-Garnett (DMG) theory was used to model the optical properties of these Au nanoparticle-containing membranes, The trend of a redshift in lambda(max) for the plasmon resonance band with decreasing aspect ratio (observed previously for pn,late particles) is observed over this entire aspect ratio range. Indeed, the redshift with decreasing aspect ratio becomes much more dramatic for the oblate particles. Because of this enhanced sensitivity of lambda(max) to the aspect ratio, we suggest that the DMG theory can be used to obtain the aspect ratio for oblate Au nanoparticles of unknown length by matching the experimental lambda(max) with a DMG calculated value using aspect ratio as the adjustable parameter.