Optical tweezers are employed to measure the forces of interaction within single pairs of poly(acrylic acid) (PAA) grafted colloids with an extraordinary resolution of +/- 0.5 pN. Parameters varied are the concentration and valency of the counterions (KCl, CaCl2) of the surrounding medium as well as its pH. The data are quantitatively described by a recently published model Of Jusufi et al. [Colloid Polym Sci 2004; 282:910] for spherical polyelectrolyte brushes which takes into account the entropic effect of the counterions. For the scaling of the brush height a power law is found having an exponent of 0.25 +/-0.02 which ranges between the values expected for spherical and planar brushes. From the model the ionic concentration inside the brush is estimated in reasonable agreement with the literature. (C) 2008 Elsevier Ltd. All rights reserved.