The most important issue with the application of optical pyrometry is the particulate contamination of the system's lens. This results in measurement errors and seriously degrades the instrument's operational performance. This paper will illustrate the fouling mechanism, through the flow field and particle trajectory investigation of a pertinent case study, together with an analysis' of the deposits that constitute the fouling of the pyrometer lens. It will be shown that the predominant flow feature within the purging system is a swirl pattern that develops at the mouth of the unit's still tube. This feature has the ability to draw contaminant particles down onto the pyrometer lens thus significantly increasing the likelihood of particle deposition. (C) 2002 Elsevier Science Ltd. All rights reserved.