Polyelectrolyte multilayer microcapsules of two different sizes consisting of poly(allylamine hydrochloride) and poly(styrenesulfonate) assembled on melamine formaldehyde cores were investigated by means of scanning (SEM) and transmission electron microscopy (TEM), confocal laser scanning microscopy (CLSM), and scanning force microscopy (SFM) regarding their stability at temperatures above 100 degreesC. A pronounced shrinking of the capsules accompanied by a wall thickness increase by more than 10 times was observed after incubating the capsule suspensions in an autoclave at 120 degreesC for 20 min. An estimation of the capsule wall volume revealed a densification of the polyelectrolyte layers and an exclusion of intercalated water. The gain of entropy resulting from a more coiled and interdigitated state of the polyions as well as from the great decrease of the polyelectrolyte/water interface is considered to be the driving force of this rearrangement process.