The adsorption-induced surface stress and mass can cause the resonant frequency shifts of a microcantilever, which is used as the sensing mechanism for a resonator sensor. Determining the adsorption-induced surface stress and mass from the experimentally measured data of resonant frequencies forms an inverse problem. Because there are infinite combinations of surface stress and mass which can result in the same change of one resonant frequency, the previous studies usually measure surface stress or mass by one measurement method and then find the other by another different measurement method. This study shows that surface stress and mass have different impacts on the resonant frequencies of a microcantilever. Two resonant frequencies are used to uniquely determine the adsorption-induced surface stress and mass. Mathematically, the new method presented in this study provides an efficient and straightforward solution to the inverse problem and its accuracy is also demonstrated. Physically, the new method only requires the dynamic mode to measure the resonant frequencies, which should be of a great help to various sensor applications.