We report on de Haas-van Alphen measurements on the quasi two-dimensional o
rganic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) in magnetic fields up t
o 27 T. Above 16 T where magnetic breakdown occurs we observe besides F-alp
ha (closed orbit) and F-beta (breakdown orbit) also the difference frequenc
y Fbeta-alpha which cannot be explained by the semiclassical theory of magn
etic breakdown. We will discuss the experimental results, also the observat
ion of unusual effective electron masses, in the context of chemical potent
ial oscillations. (C) 1999 Elsevier Science B.V. All rights reserved.