A previous study of the relationship between the hyperfine splitting parameters of an aliphatic radical and the geometry at its tervalent carbon atom has been extended to a wider range of radicals. In particular, the significance of a(%) is discussed, data for some simple ketyls *CR1R2*0-are reported and interpreted, and it is suggested that useful information is available from the ratio a(Me) :a(H) both for radicals *CHMe-X and for the pairs of radicals *CMeXY and *CHXY. It is noted that, in radicals of the type *CHY*CH,X (X = an electronegative substituent, Y = OH or OR),a(CH,) is unusually low; it is argued that this is a manifestation of an interaction which is associated with an eclipsed conformation of the C-X bond and the half-filled orbital and also with less deformation from coplanarity a t the tervalent carbon atom than in the absence of X. Finally, INDO calculations for some small cyclic radicals support the conclusion from the experimental data that the species are not coplanar a t the tervalent carbon atom.WE have previously presented evidence that, in organic radicals in which the tervalent carbon atom is attached to at least one hydroxy-or alkoxy-substituent, the three bonds to that carbon atom are not coplanar.2 (For simplicity, we refer to such species as ' bent '.)The degree of bending increases with the number of such substituents and is also dependent, for cyclic radicals, on the size of the ring. It was suggested that the capacity of the oxygen-containing substituents to induce bending is related to their +M effects; thus, +-conjugation leads to an increase in the electron density a t the tervalent carbon atom, and bending occurs to reduce the resulting inter-electronic repulsions. The dependence of bending on ring size was attributed to increase in the 9 : s ratio of the ring bonds involving the tervalent carbon atom as the strain in the ring is increased. The results of INDO calculations for two of the simpler radicals were in reasonable agreement with the conclusions from the experimental data,We have now extended our study of radical geometry to a much wider range of substituted alkyl radicals. In particular, first, we have examined the significance of a(a-13C) in the light of data for a more extensive list of radicals than was available before. Secondly, we have explored in greater detail the possibility, previously hinted at,2 that for radicals of the type -CHMe*X the ratio of the hyperfine splitting constants for the (p) methyl protons and the single ( a ) proton can give information about the hybridisation of the tervalent carbon atom; 7 we have concluded that it can do so, and this in turn has enabled us to discuss further the factors that determine the hybridisation of this carbon atom, and hence the shape of the radical, and to attempt to separate the dependence of proton-splitting constants on delocalisation within the radical from their t a, p, y Refer respectively t o the tervalent carbon atom and