We report calculations of the vibrational spectrum of muH(2)/HD/DT as well as D-3 in their first-excited electronic state, both with the inclusion of the geometric phase effect and without including it. The results show that, especially for muHD and muDT, it plays a minor role for the first 30 vibrational levels of such systems. This can be rationalized from the tilting of the C-3-axis due to mass effects. A simple phenomenological criterion for the relative role of the GP effect in tri-hydrogen isotopomers has been suggested. (C) 2002 Elsevier Science B.V. All rights reserved.