A constant-concentration, constant-capacitance, macrohomogeneous porous-electrode model is used to investigate the effects of side reactions on the charging and cycling of electrochemical double-layer capacitors. A porous carbon capacitor with sulfuric acid electrolyte is a typical system, and corresponding physical properties are used to illustrate the effects. Oxygen and hydrogen evolution are considered the dominant side reactions, and a Tafel form for the kinetic expressions is assumed. It is shown that keeping the cell potential within the thermodynamic stability window of the solvent does not guarantee negligible losses, even in the kinetics are not particularly facile. Losses are substantial at early cycles and decrease to a constant value at later cycles; this value can be significant. The dependences of coulombic an energy losses on the maximum cell potential, electrode thickness, and discharge rate are discussed. In a poorly designed cell, the energy lost to side reactions can be as large or larger than the ohmic loss; hence consideration of side reactions is essential for good design.