Extinction coefficients of small dodecanethiolate-stabilised gold nanoparticles in the range 1.95-3.77 nm were determined for the first time in a systematic study using transmission electron microscopic and UV-vis spectroscopic analyses. Six polydisperse nanoparticle systems were prepared by a modified Brust-Schiffrin procedure and studied as isolated solids following purification. Clear relationships between extinction coefficient (epsilon*, in mL mg (1) and epsilon, in dm(3) mol (1) cm (1)) and mean nanoparticle core diameter (D, in nm) have been established. The observed relationships permit the prediction of concentration or size of unknown nanoparticle systems simply by recording the absorbance at a single fixed wavelength. (C) 2008 Elsevier B.V. All rights reserved.