In addition to adsorption, the main limitation in dead-end ultrafiltration with inside/out hollow fibres is the formation of an external deposit on the membrane surface. The structural properties of the deposit, like its thickness or porosity, control filtration performance. Thus, local information from in situ, real-time measurements are required to characterise the basic mechanisms involved in deposit build-up. For that purpose, time variations of the local thickness and porosity of deposits were studied in situ using a previously developed optical method (J. Mendret, C. Guigui, R Schmitz, C. Cabassud, R Duru, An optical method for in situ characterisation of fouling during filtration, AlChE J. 53 (9) (2007) 2265-2274.). This local characterisation was performed during dead-end ultrafiltration in a confined geometry resembling in/out hollow fibres with regard to the global operating parameters (flux and pressure). The aim of the study was to better understand the relationship between deposit structure and process performance. The results obtained allowed the mechanisms of cake growth under pressure to be better understood. Two different stages were observed during the deposit thickness growth: first a thin and resistant layer develops and then a thicker and more permeable layer appears. From the local characterisation, it was possible to detect a limit value of deposited matter for the observation of pressure effects on the deposit thickness. Below this value, the structure of the deposit was the same whatever the pressure: a thin and incompressible layer. Above this limit, the deposit was compressible and its thickness and porosity depended on the pressure. (C) 2009 Elsevier B.V. All rights reserved.