The mass sensitivity distribution of the quartz crystal microbalance, denoted S(r,8), was determined in situ for pianopiano and plano-convex Aleut quartz resonators. The method involved measuring the frequency change associated with localized mass deposits created by electrochemical deposition using a scanning microelectrode in close proximity to the QCM eiectroded surface. Complete mathematical descriptions of S(r,8) were obtained from the measured data under conditions commonly encountered in QCM experiments. Our experimental resuits suggest that crystal contouring increases sensitivity toward the resonator center while reducing the extent of field fringing. Increaslng solution viscosity reduces the sensitivity to mass changes in the center of the resonator and increases field fringing, while an increase In the rigid mass within the eiectroded area increases the sensitivity in the center of the resonator. Under most conditions, S(r,8) appeared dependent upon electrode geometry and tab placement but exhlbited neg#glM dependence upon the crystal orientation with respect to the shearing axis. The integral sensitivity, as determined by integration of the sensitivity maps, was significantly less than that predicted by the Sauerbrey equation. These resuits demonstrate that detailed understanding of the sensitivity distribution is important when the QCM is used in liquid media, In the presence of nonuniform mass deposits, and under conditions favoring field fringing