This article deals with the basic mechanism of aging, that is water treeing (WT), that takes place in polymer-insulated power cables. WT was studied by means of computer-aided video-enhanced microscopy, microspectrophotometry, and micromanipulation. The microdiagnostics procedure was used for cables operating in a wet environment. A phenomenological WT model aimed at estimating the residual lifetime, which considers WT as a diffusion with-reaction process, was developed. The theoretical calculations were in reasonable agreement with the experimental data. (C) 2003 Wiley Periodicals.