Ultrafine particle sizes may be measured over a limited range with an ultrafine condensation particle counter (UCPC) by analyzing photodetector pulse heights. The pulse height analysis (PHA) technique has been employed successfully in our laboratory with a prototype UCPC. However, attempts to incorporate PHA into a commercial UCPC (the TSI model 3025) met with initial lack of success, yielding anomalous and irreproducible pulse height spectra. Operating parameters and subsystems of the commercial UCPC, including condenser/saturator temperatures, aerosol flows, optical system (light source and detector), and post-photodetector electronics, were examined for contributions to the observed pulse height spectra. The optical system, specifically the light source and scattering geometry, was found to be the cause of the anomalous pulse height performance. Replacing the existing 90° scattering, laser light-photodiode detection system with a forward-scattering, white halogen light source and photodiode from an older condensation particle counter (the TSI 3020) resulted in satisfactory pulse height response from the instrument, allowing pulse height analysis with the commercially available UCPC.