A small-angle X-ray scattering (SAXS) study of micelles formed by a symmetric block copolymer consisting of poly(styrene) and poly(ethylene oxide) (molecular weight of each block : 1000 g/mol; concentration up to 15.4 wt %) is given. The contrast of the micelles toward water was changed by adding glycerol to the solution. The analysis of the SAXS data shows that the addition of glycerol does not change the structure of the micelles. With the aid of contrast variation, SAXS allows determination of the following integral parameters : (i) the mean electron density of the particle <(rho)over bar> together with the particle volume V-p, (ii) the radius of gyration at infinite contrast R(g,infinity) together with the inhomogeneity parameter alpha, and (ii) the volume V-c related to the shape of the particles as well as parameters characterizing the internal density fluctuation of the particles. Furthermore the correlation length l(c) has been determined as a function of concentration and contrast. A comparison of these parameters and of the scattering intensities at various contrast with concentric models of the radial electron density shows the following : The micelles have an overall spherical shape and a narrow size distribution (weight-average aggregation number = 400; standard deviation of the size distribution 10.9%). The unpolar component poly(styrene) together with a fraction of the polar poly(ethylene) blocks is located in the core.