Nuclear microprobe methods have been developed to measure light element and metallic poison distributions in used catalysts. The methods are based on the irradiation of sectioned catalyst pellets with microbeams of energetic (MeV) charged particles. Irradiation with deuterons allows the determination of carbon and nitrogen by nuclear reactions, and vanadium, nickel, and other elements with atomic number >12 by particle-induced X-ray emissions. Hydrogen can be determined by an elastic recoil analysis method based on irradiation with helium ions. The development of the methods for catalyst applications is described. All elements can be determined at concentrations of <0.1 wt% at spatial resolutions as high as 5 mu m. The methods have been applied to examine hydrotreating and hydroconversion catalysts used to treat heavy petroleum fractions. The extent of intra- and interpellet variations in deposited elements has been found to be high, but statistically acceptable average distributions can be obtained by examining no less than five pellets per batch. The interrelationship between deposited elements has been examined. An observation is that the H/C atom ratio of coke falls in regions of pellets containing high concentrations of vanadium. Early work indicates that nitrogen compounds tend to form coke nearer the surface of pellets than hydrocarbons. Information has been obtained on pore filling.