Co-ions, dissolved atmospheric gas, and buffers are shown to play an apparently remarkable role in colloidal particle interactions. The effects are exhibited in flocculation studies on suspensions of charged solid paraffin particles. The effects cannot be explained by present theories of colloid science. In the standard theory, co-ions, electrolyte ions of the same charge as a colloid particle, ought to exhibit little or no qualitative variation in their effects on the stability of suspensions. Dissolved gas has been assumed to have no role, and buffers have always been thought to influence interactions via pH alone. In this work, reasons that theory is inadequate are advanced.