The pumping speed for nitrogen of a Bayard-Alpert ionization gauge was measured in an ultra-high-vacuum system with pressures between 10 −9 and 10 −7 mmHg. The highest pumping speed measured was 2l/sec, mainly chemical or non-electronic. After about 10 15 molecules of nitrogen had been pumped, the chemical pump had decayed leaving the pumping speed about 0.25l/sec, almost entirely electronic, at 8 milliamperes electron current. This speed was approximately constant until nearly 10 17 molecules had been pumped, when a major decline in electronic pumping speed was observed. There was no measurable re-emission of chemically pumped nitrogen, and the re-emission probability of electronically pumped nitrogen was 10 −6 to 10 −5/sec about 1 hr after completion of pumping. It is concluded that the main mechanism contributing to chemical pumping is the formation of a second adsorbed layer on the metal parts of the gauge, in particular the grid.