The photoelectric condensation nucleus counter of Pollak with convergent light beam has been compared with an electrical aerosol detector using monodisperse aerosols with particle diameters between 0.025 and 0.15 μm, particle concentrations between 127 and 260,800 cm−3, and particles of two different chemical constituencies, e.g., NaCl and material volatilized from a heated nichrome wire. Very good agreement has been obtained. The discrepancy between these two methods was found to be less than 9% at concentration levels below 104 particles cm−2 and 17% at 2.5 × 105 particles cm−3. This discrepancy is well within the combined uncertainties in the two independent aerosol concentration measuring methods.