Abstract We have prepared gold sols with mean particle diametersdmin the range 4 to 50 nm and measured their extinction spectra and size dependences of the extinction peak position λmaxand valueEmax. The measured increasing function λmax(dm) displays a pronounced bend near the particle diameterdm∼ 10 nm, where the value of λmaxsharply decreases with reduction in the particle size. To explain these findings, the extinction spectra of sols with the particle size and axial ratio polydispersity are calculated using Mie's theory, the T-matrix method, and various experimental sets of the bulk gold optical constants modified with regard to size-limiting effects. It is shown that the measured λmax(dm) andEmax(dm) dependences are inconsistent with calculations based on Mie's theory and the bulk gold optical constants. The most generalized model including the size dependence of the imaginary part of the dielectric permeability and the size and shape polydispersity gives good agreement with experimental extinction spectra for 5-, 10-, 24-, and 40-nm sols, as well as with the measured functions λmax(dm) andEmax(dm). Based on electron-microscopic and spectral data, calibration curves are obtained for efficient spectrophotometric control over the particle size and for estimation of the amount of restorer essential for the preparation of particles of a given size. A simplest two-layer spherical model is employed to elucidate the basic changes in sol spectra after conjugation with specific biomacromolecules and to draw some conclusions about the conjugate shell structure.