The self-discharge behaviour of supercapacitors is an important factor when considering their suitability fur some applications. In this paper, measurements of the self-discharge rates of carbon-based supercapacitors with organic electrolytes are presented and interpreted in terms of two mechanisms. The first is the diffusion of ions from regions of excess ionic concentration formed during the charging of the capacitor and the second is leakage of charge across the double-layer at the electrolyte-carbon interfaces in the capacitor. The dependence of the self-discharge rate on temperature and on the initial voltage across the capacitor is described.