In the present Letter, we have studied the Rydberg spectroscopic behaviour of the CF3Cl and CF3Br isovalent molecules. Absorption oscillator strengths for both molecules have been calculated with the molecular-adapted quantum defect orbital (MQDO) approach. We have sought and found important similarities between the spectral intensities of analogous transitions ill these isovalent molecules. Further similarities with the spectra of their isolated halogen atoms has not only served the purpose of assessing the quality of our calculations, but may also offer some relevant practical use. (C) 2005 Elsevier B.V. All rights reserved.