We describe an x-ray photoelectron spectroscopy spectrometer capable of determining the composition, thickness, and chemistry of thin film surfaces during growth. The instrument operates at pressures up to a few millitorr, an adequate range for many sputtering, low pressure chemical vapor deposition and thermal evaporation processes. Using a conventional x-ray source, spectra of individual elements can be rapidly accumulated in 10-30 s. To illustrate its capabilities, we present two examples of measurements made with the spectrometer. The low pressure, chemical vapor deposition growth of a WO3 film on Si in a 1 mTorr O-2 environment, and the thermal oxidation of Si in a 2 x 10(-4) torr O-2 environment. We believe these represent the first measurements of such processes in real time by electron spectroscopy. We also describe the spectrometer construction and capabilities in some detail, contemplated improvements in its performance, and anticipated applications.