A simple method of characterization of suspensions of spherical nanoparticles with monotonically variable size is proposed. It allows for the in situ measurement of the particle size as well as spectral dependence of their refractive indices. The method requires three optical channels: one for the illumination of a suspension by white light and two for the measurements of the spectra of scattered light. Parameters of the particles are determined by fitting the measured temporal spectral surfaces by the calculated Mie scattering functions. The method is applied to the particles being grown in a low-pressure reactive plasma of a discharge in an acetylene-argon mixture