The CSIRO Mineral Resources X-ray technologies group develops industrial X-ray fluorescence (XRF) instruments designed to measure the concentrations of precious and base metal such as gold, platinum, nickel and copper in mineral slurries. While XRF is a well-developed technique, there are still many challenges when measuring in-situ non-homogeneous samples, for example slurries in a mineral processing plant. One of the main challenges of measuring minerals in a slurry sample is that the XRF results are affected by the size of the particles in the slurry, known as the particle size effect. The intensity of characteristic fluorescent radiation from mineral phases in a slurry is affected by the particle size of the ore being measured. This size variation of the ore particles can lead to substantial analysis error. In extreme cases, the XRF response from a given element can vary more than 50% between samples of the same concentration with varying particle size distributions. These large uncertainties significantly impact the usefulness of XRF as a tool for on-stream elemental analysis for mineral slurries. For laboratory XRF applications, the particle size effects are minimised using appropriate sample preparation techniques. Powders can be milled to sub-micron sizes, homogenised using the glass fusion method, or dissolved to form a solution. These techniques can be time consuming and costly and are not available when measuring samples using in-situ or with a portable XRF. However for online XRF measurements of slurries, there is currently no solution to account for changes in particle size. Instead, to combat changes in particle sizes and changes to the matrix material of a slurry, current XRF analysers require regular calibration, which is time consuming and costly to the processing plant. CSIRO have developed and patented an XRF technique that can identify variations in particle size distributions in slurries, as well as variations in mineralogy for a given element of interest. The technique requires only a small adjustment to a traditional XRF system, and could be incorporated into a range of XRF on-stream analysers irrespective of their configuration within a plant. The goal is to integrate the particle size correction technique into an XRF analyser that includes a combination of improved matrix material corrections and particle size corrections so that it can be used for a variety of applications without the need for regular re-calibration. CSIRO has measured how the XRF results from a slurry analyzer vary as the average particle size of the slurry changes. The results are shown in Figures 1 and 2.