Atomic force microscopy (AFM) has been used to measure the force of interaction between pairs of silica colloids in aqueous inorganic and natural organic electrolyte solutions. In solutions containing a mixture of mono- and divalent inorganic ions, the force measurements on approach can be understood in terms of an electrical double-layer interaction, with decay lengths being comparable to the predicted Debye length. The force of interaction on separation in these mixed electrolyte solutions has been found to be entirely repulsive. Force measurements in a mixture of monovalent ions and natural organic matter (NOM), which is a common component of natural waters, are also reported. The decay length of the electrical double-layer interaction is observed to decrease upon addition of NOM. Force measurements on separation are characterized by pH-dependent adhesion, which has been interpreted in terms of specific surface adsorption and the intermolecular interaction of NOM. A comparison with particle micro-electrophoresis data for the silica colloids in the mixed electrolyte solutions has also been made.