K-Ar and Rb-Sr isotope studies of some rocks from the Mooirivier Complex indicate severe disturbance by processes postdating the formation of the complex. According to K-Ar dating of biotites from the Spreetshoogte Pass area which yielded individual ages between 330 and 545 Ma, such processes must also postdate the Damaran Orogeny. It is furthermore shown that the open system behaviour of the analysed biotites involved a severe loss of argon and potassium and a gain of atmospheric argon. In this context it is shown how reference lines in 40K/36Ar vs. 40Ar/36Ar and 40K vs. 40Arrad diagrams which are defined by data points affected by the loss of argon and potassium and the gain of atmospheric argon may lead to incorrect interpretations. Rb-Sr data obtained for amphibolites and granitic rocks from Mooirivier 160, yielding ages of 983.4 ± 97.0 Ma and 1141.2 Ma, respectively, indicate a possible disturbance of the Rb/Sr systems in this area by late Mokolian intrusives, while the whole-rock Rb-Sr data of samples from the Spreetshoogte area do not provide meaningful ages.