Direct mass measurements of short-lived Cs and Ba isotopes have been performed with the tandem Penning trap mass spectrometer ISOLTRAP installed at the online isotope separator ISOLDE at CERN. Typically, a mass resolving power of 600 000 and an accuracy of m 13 keV have been obtained. The masses of 123;124;126 Ba and 122m Cs were measured for the rst time. A least-squares adjustment has been performed and the experimental masses are compared with theoretical ones, particularly in the frame of a macroscopic-microscopic model.