Investigations on the influence exerted by magnesium on the superconducting properties and phase composition of samples from the Bi-Mg-Sr-Ca-Cu-O system are reported. The synthesized materials are analyzed by X-ray diffraction, emission spectral analysis, scanning electron microscopy, and electrical resistance measurements as a function of temperature. It is found that the addition of magnesium results in changes in the phase composition of the materials. The critical temperature is in the range of 93-95 K. The addition of Mg in certain optimum concentrations can have a positive effect on the superconductivity of the Bi-Sr-Ca-Cu-O system. © 1991 Plenum Publishing Corporation.