Lead magnesium niobate (Pb(Mg1/3Nb2/3)O-3, or PMN) is a relaxor ferroelectric with many potential applications, As a relaxor ferroelectric, PMN displays a diffuse phase transition and its temperature of maximum relative permittivity (T-m) shifts to higher temperatures as the frequency increases. When 10 mol% of lead titanate (PbTiO3, or PT) is added, a solid solution is formed with a T-m value near ambient temperature. These properties can be manipulated to serve many functions, such as applications in transducers, actuators, and active vibration-control systems, The purpose of this experiment was to examine the effect of lanthanum doping on the microstructure and dielectric properties of a 0.9PMN-0.1PT solid solution. Lanthanum was added in concentrations of 0.0, 0.5, and 1.0 mol%. The samples were fired under different lead atmosphere conditions, to examine the effect of lead depletion. The presence of lanthanum had no effect on the grain structure of the fired samples; however, the grain size did increase when lend zirconate (PbZrO3) was added to the system to maintain a high partial pressure of lead. The addition of lanthanum also created a weak grain-boundary phase, which led to intergranular fracture. Lanthanum doping decreased the maximum permittivity by similar to 17% and decreased the T-m value by 10 degreesC far every 0.5 mol% of lanthanum that was added. The. diffuseness of the permittivity -versus-temperature curve was described using both the fractional power parameter gamma and the parameter delta, a la Smolenskii. Both parameters increased as lanthanum was added to the system, which indicated a broader transition. Lanthanum addition also decreased the high-field polarization but did not have a conclusive effect on induced microstrain, Aging was not observed under any of the examined conditions.