jats:pIn this letter, we experimentally demonstrate the existence of an optimal operating region in a resonant microelectromechanical systems sensor utilizing the principle of vibration mode localization, to achieve ultra-high resolution amplitude ratio measurements. We have shown analytically that the coupling strength and stiffness mismatch between the two resonators are primary parameters that influence the resolution of the amplitude ratio measurements within such a sensor. Using this optimization strategy, a minimum noise spectral density equivalent to 16 ppb/Hz1∕2 in terms of normalized stiffness perturbation is experimentally demonstrated. This is by far the best resolution achieved in this type of sensor. These results can be used to aid the design of future high resolution sensor employing mode localization.</jats:p>