The utility of in situ X-ray absorption spectroscopy (XAS) in determining structural parameters, through analysis of the extended X-ray absorption fine structure (EXAFS), and electronic perturbations, through a white line analysis of the X-ray absorption near edge structure (XANES), is demonstrated for Pt/C. PtRu/C and PtMo/C fuel cell electrodes. The results provide verification that the enhancement of CO tolerance of the alloy catalysts occurs via an intrinsic mechanism for the PtRu alloy, whilst a promotion mechanism is in operation for the PtMo alloy. Preliminary results of an in situ powder X-ray diffraction (XRD) method which utilises synchrotron radiation (SR) and a curved image plate detector are also presented, using Pd/C as an example. The lattice expansion upon formation of the P-hydride is clearly observed.