Development of a novel in situ composition sensor for thin films, based on X-ray fluorescence (XRF), is described. Design and verification of equipment and analysis techniques are presented. Results from in situ analysis and control of CuInxGa1-xSe2 (CIGS) thin films are shown. The work presented includes several advances for in situ sensing of thin-film deposition. Novel elements of the XRF hardware include protection of the sensor from the deposition environment, use of a sensor-to-sample distance appropriate to deposition chambers, and the use of only low-cost components operating at room temperature. Novel aspects of the XRF analysis include one-sample calibration that gives valid results over a wide range of compositions, real-time CIGS analysis, and compensation for variations in substrate location and X-ray tube current drift by using the substrate signal. The use of XRF as a sensor for real-time closed-loop control of deposition is also novel.