Differential cross sections have been measured for Na scattered by the tetrahedral molecules SiCl4, C(CH3)4, Si(CH3)4and Sn(CH3)4in the thermal energy range. Due to the high velocity resolution (Δg/g≈4 per cent) the rainbow structure with primary and at least one secondary rainbow has been resolved. The broadening and quenching of this structure has been used for the determination of the anisotropy of the interaction potential. The corresponding calculations are based on two different formulations of the sudden approximation. It is observed that due to a compensation of repulsive and attractive forces the rainbow scattering determines an effective anisotropy, αeff, which is found to increase for the tetramethyl molecules from 0·2 to 0·7 with increasing size of the central atom, whereas the value for SiCl4is 0·9 in contrast to the behaviour of other tetrachlorides.