A drift tube-mass spectrometer system employing Bradbury-Nielsen shutters has been used to measure the mobility of Li + ions in He at 294 and 80 K and Li + ions in Ar at 294 K. The E/N range used was 3 to 80 Td (1 Td == 10 - 21 Y cm2). The zero field reduced mobility for Li + in He was found to be 22·81±0·11 cm2 y-1 s-I at 294 K and 19·64±0·29 cm2 y-1 s-I at 80 K. The value for Li+ in Ar at 294 K is 4·66±0·22 cm2 y-1 s-I. The reduced zero field mobility for the cluster ion Li +. He in He at 80 K and low values of E/ N was found to be 14·84 ± 0·22 cm2 y -I s - I. The equilibrium constant for the formation and dissociation of Li + . Ar cluster ions at 294 K was obtained by fitting to the variation of the measured mobility with gas number density at low E/N values. The value obtained, corresponding to thermal equilibrium at 294 K, was (4±0.5)xlO- 19 cm3 .