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The frequency stability of single-crystal silicon microelectromechanical systems resonators encapsulated with epitaxial polysilicon (epi-seal) is investigated. As silicon resonators have significant temperature dependence, the inherent frequency stability of the resonators is masked by temperature-induced noise. Using two resonators side-by-side and assuming identical temperatures and fluctuations, temperature effects are eliminated, resulting in the two resonators tracking each other within ±10 ppb, or 3 × 10-4 °C, over a month. Power and thermal cycling the resonators produced no observable effects on the resonant frequency. This result indicates that silicon resonators make excellent on-chip thermometers, or high stability timing references if temperature is compensated well.