We present the development and performance of a new, reactor-like, in-house ambient pressure X-ray photoelectron spectrometer (AP-XPS) using Al Kα, which can study material surfaces, particularly surfaces of catalysts during catalysis and under reaction conditions. This ambient pressure X-ray photoelectron spectroscopy technique uses an affordable bench-top X-ray source, monochromated Al Kα, making it available for research groups on any campus. A unique feature of this in-house AP-XPS is the integration of a micro ambient pressure reaction cell working in either flowing or batch mode into a monochromatic Al Kα source and an ambient pressure energy analyzer. This integration allows XPS studying surfaces of materials while they functionalize in a flowing gaseous environment. Another feature is the integration of characterization of surfaces of catalysts into simultaneous measurements of catalytic performance by using on-line, quadrupole mass spectrometry and gas chromatography. Performance tests of this in-house AP-XPS have demonstrated that it can study material surfaces at temperatures up to 500–550 °C in a gaseous environment with pressures up to 25–50 Torr. This design has successfully brought the synchrotron-based AP-XPS technique to research groups on campuses.