IT seems possible to account for the steric effect of non-reacting groups on substitution rates on the lines of the following scheme, illustrated here by the example of the symmetrical substitution of t-butyl chloride by chlorine ions: In the transition state, the four t-butyl carbons will be co-planar and the methyl groups will cause least obstruction if arranged in either of the two symmetrical positions one of which is shown in Fig. 1. This is represented in terms of a Fisher model in which the carbon atoms have the usual covalent radii of 0.77 A. and the van der Waals' radii of 1.5 A., while the corresponding values for the hydrogen atoms are 0.32 A. and 1.2 A. respectively. The valencies of the central carbon atom are co-planar, with an angle of 120° between them ; the valencies of the other carbon atoms are at the tetrahedral angle. The position of the chlorine particles in the transition state will be such that the line joining their centres to that of the central carbon will be perpendicular to the plane of the carbon atoms. Further, the carbon chlorine internuclear distances will be equal. From the latest calculation of the transition state, this carbon chlorine distance has been found to be 2.3 A., which is practically equal to the sum of the covalent radius of carbon and the ionic radius of chlorine. These chlorine particles wilt have a van der Waals' radius of 1.8 A.