The sizing capability of an ultrafine particle condensation nucleus counter (which uses butanol as the condensing fluid) equipped with pulse height analysis was evaluated in terms of particle composition for sulfuric acid aerosol and sulfuric acid aerosol to which gas-phase ammonia had been added. The response of the counter depended on composition for a range of particle sizes when the water partial pressure was low. For water partial pressures <5 Torr and for particles >4 nm in diameter, the response (pulse heights) of the instrument to particles of a given size was substantially different for sulfuric acid particles and those that were neutralized with ammonia. For water partial pressures >5 Torr, however, neutralizing the particles with ammonia had little effect on pulse height distributions. For particles smaller than 4 nm diameter the pulse heights were insensitive to exposure to ammonia.