A new on-line mass spectrometer system was constructed which enabled the mass signal-potential profile to be obtained on a small single-crystal electrode in the dipping state. The gas inlet for the mass spectrometer was a pin-hole (a few micrometers in diameter) covered with a Teflon film and in contact with the center of the electrode surface. This system resulted in a high signal-to-noise ratio. No modification of the cell was required. The new instrument was used to investigate the electroreduction of acetylene and showed that Pt(110) has a high activity compared with Pt(111) and Pt(100).