The well-established atomic force microscopy (AFM)-based colloidal probe technique (CPT) and optical tweezers (OT) are combined to measure the interaction forces between blank SiO2 surfaces in aqueous ionic solutions (CaCl2) of varying concentration at PH 7. Spherical colloids (SiO2, diameter similar to 4.63 +/- 0.05 mu m) taken out of the same batch are used by both methods. In the case of CPT, a single colloid is glued to a cantilever, and the interaction forces with a plain SiO2 surface are determined in dependence oil I he concentration of the surrounding medium. For the OT studies, two colloids (one fixed to a micropipet by capillary action, the other held with the optical trap) tire approached to each other in nanometer steps. and the resulting forces are measured for the same media as in the CPT experiment. Both techniques fit well to each other and enable one to cover interaction energies ranging from 10(-5) to 1 mN/m. The experimental data are well described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory revealing that the effective surface charge density changes slightly with concentration.