The performance of electron refrigeration by means of tunnel junctions between superconducting and normal-metal electrodes is studied theoretically. A suitable approximation of the basic expression for the heat current across the tunnel junctions allows the investigation of the main features of the device such as its optimal bias voltage, its maximal heat current, its optimal working point, and the maximal temperature reduction. The theoretical results are compared with those of a recent experiment. ©1997 American Institute of Physics.