In this paper, the fouling of sintered -alumina membranes by an uncontaminated semisynthetic metalworking fluid (MWF) is addressed. Experimental evidence of the form of flux reduction curves, scanning electron microscope images of the membranes, and MWF particle size measurements is used to identify two fouling mechanisms, pore blocking and partial pore blocking, as the major contributors to flux decline. A probability-based mechanistic model is developed based on the time-dependent particle size distribution and membrane pore sizes. The model is fitted to experimental data from two commonly used membrane pore sizes with good agreement. Partial blocking is shown to be a predominant first step in the pore blocking mechanism in microfiltration of semisynthetic MWFs due to the tortuous nature of the pores present in sintered ceramic membranes.