This paper describes a technique for investigating the fouling of microfiltration membranes by measuring the pressure drops across two membranes fed in series by a constant rate pump, which enables a distinction to be made between surface fouling and internal fouling of the membrane. In the case of the microfiltration of BSA solutions, the technique shows how the type of pump and the operating temperature influence membrane fouling and how protein denaturation and adsorption give rise to different types of fouling. The technique is also used to investigate the microfiltration of beer and shows how this is affected by membrane properties and the aging of beer.