Fluid catalytic cracking, an important process in the chemical industry, uses porous composite particles to convert the heavy fractions of crude oil into transportation fuels and chemical feedstocks. The employed particles, a spherical composite of zeolite and clay, decrease in catalytic activity during long-term operation, which demands their continuous replacement. To extend the lifetime of these catalysts, it is essential to understand the structural changes that cause the observed decrease in activity. By using a range of electron microscopy and elemental mapping techniques, the structural and chemical makeup of pristine and progressively deactivated catalyst particles were characterized from the micro- to the nanometer scale. Independent of the deactivation degree, the catalyst particles are found to maintain a porous interior structure similar to that of the pristine catalyst. An increasingly dense amorphous silica-alumina envelope enwrapping individual catalyst particles is formed during operation. Regions that contain calcium compounds and spinel-type iron oxide particles are present. While the porosity of this envelope apparently decreases with deactivation, its thickness reaches a plateau at about two micrometers. This process happens not only independently of the detected impurity levels, but moreover, the uptake of impurities appears to be physically halted by the forming envelope.