We study the deformation and stability of a series of gold/polysilicon MEMS plate structures fabricated by the MUMPS surface micromachining process and subjected to uniform temperature changes. We measured, using an interferometric microscope, full-field deformed shapes of a series of circular gold (0.5 μm thick)/polysilicon (1.5 μm thick) plate structures of diameters ranging from D = 150 to 300 μm. From these measurements we determined the pointwise and average curvature of the deformed plates. Although the curvature generally varies with position, the deformation response of the plates can be broadly characterized in terms of the average curvature as a function of temperature change. In terms of this, three regimes were observed: i) linear thermoelastic response independent of plate size, ii) geometrically nonlinear thermoelastic response that depends on plate size, and iii) bifurcations in the curvature-temperature response that also depend on plate size. We modeled the deformation response both analytically and with the finite element method. Good qualitative and quantitative agreement was obtained between predictions and measurements in all three deformation regimes, although the details of bifurcation are less accurately predicted than the linear and nonlinear response. This is attributed to their strong sensitivity to slight imperfections. Good agreement was also obtained between measurements and predictions of the spatial nonuniformity of the curvature across the plate.