To establish the mechanism of charging of solute entities in nonaqueous solvents, the number density of charges must be measured. Conductivity measurements alone are not always sufficient. Here we explore the use of total internal reflection microscopy (TIRM) to determine the Debye length and ionic strength by measuring the potential energy profile between a charged microscopic sphere and a charged plate separated by a solution of 0.1-2 mol center dot m(-3) KOH dissolved in a mixture of acetone with 10-20% v/v water as a model nonaqueous solvent. KOH behaves as a weak electrolyte in these solvents. The dissociation constant was determined to be 1.99 mol center dot m(-3) in the 15% v/v solution by both conductivity and TIRM experiments. Conductivity is much preferred for determining the concentration of charge in solutions like this in which the mechanism of charging is simple dissociation of ion pairs. But in nonpolar solvents, in which the mechanism might be proton transfer between two neutral micelles, TIRM can yield the number density of charges while conductivity alone cannot.