In this and the following paper we present scattering studies on binary colloidal mixtures made of charge-stabilized polystyrene (PS) and perfluorinated (PFA) particles with diameters sigma=79 and 162 nm, respectively. Both colloidal species were mixed to well-defined compositions. The total concentration was about 9 vol % fraction. By using ultra-small-angle X-ray scattering, we directly obtained the partial scattering intensities of only the PFA particles in liquidlike ordered suspensions. Furthermore, after dividing intensities by the PFA particle form factor P-PFA(Q), We got the partial structure factor SPFA-PFA(Q) Without any additional treatment. The experimental results are compared with theoretical predictions obtained from the pure repulsive DLVO potentials and the hypernetted chain (HNC) integral equation, as applied to charged colloidal mixtures. It is shown that all measured intensities and extracted structure factors are in good agreement with the theoretical results.