Hyper Rayleigh scattering in solution is used for the determination of the first order hyperpolarizability b(colloid) of gold colloids in the size range 5–22 nm; these hyperpolarizabilities are found to range from 0.60 × 10–25 esu for 5 nm diameter colloids to up to 16.6 × 10–25 esu for 22 nm diameter colloids at the second harmonic wavelength of 532 nm; the strong size dependence of the hyperpolarisabilities reported here suggests that hyper Rayleigh scattering experiments can efficiently be used to calibrate the particle diameter of colloidal suspensions.