Three methods-scanning mobility particle sizer (SMPS), transmission electron microscopy (TEM), and diffusion charging (DC)-for estimating aerosol surface area were evaluated and compared. The aerosol used was monodisperse silver particles, having morphologies that range from spherical to agglomerated particles, with corresponding fractal dimensions from 1.58 to 1.94. For monodisperse silver particle agglomerates smaller than 100 nm, the DC response was proportional to the mobility diameter squared, regardless of morphology. For particle sizes from 80 to 200 nm, the DC response varied as the mobility diameter to the power 1.5. The projected surface area of agglomerates analyzed by TEM agreed well with that estimated from particle mobility diameters for particles smaller than 100 nm. The surface area of monodisperse particles, measured by DC and SNIPS, was comparable to the geometric surface area below 100 nm, but in the size range of 100-200 nm, the methods used underestimated the geometric surface area. SNIPS, TEM, and DC-based measurements of surface area were in good agreement with one another for monodisperse aerosol particles smaller than 100 nm.