The principles of x-ray fluorescence (XRF), and differences between the use of XRF as anin-situcomposition sensor in CIGS module fabrication and the use of XRF in typical applications, are described. It is demonstrated for measurements on CIGS samples how a number of conditions, including interelement effects and composition gradients, may complicate conversion of XRF signals to composition. Factors controlling the precision of the measurement are enumerated. ©1999 American Institute of Physics.