An empirical analysis of c.d. data (n→π*; ca. 290 nm) for a wide variety of ketones of the ‘extended decalone’ class has led to sets of numerical contributions for ring systems (Figure 8; Tables 9 and 14) and alkyl substituents (Table 3). These contributions can be summed to give Δε values corresponding very closely to those observed (usually within ±0·2 units, although a few exceptions are noted and discussed). This is the first such analysis to embrace, within a single scheme, compounds of both the trans- and cis-decalone types, as well as their polycyclic analogues. The treatment covers data obtained for solutions in four types of solvent: hexane, dioxan, aceto-nitrile, and methanol (or ethanol). Further analysis of c.d. data for extended decalones indicates that certain coplanar zig-zag arrangements of C–C bonds (termed ‘primary zig-zags’: Figure 3) are probably responsible in many cases for a major part of the observed c.d., although other significant contributions may come from alkyl substituents in the vicinity of the carbonyl group. Atoms or bonds which form part of the alicyclic framework but do not lie on or adjacent to a primary zig-zag generally make little or no contribution to the value of Δε; structural features lying very close to the carbonyl group and in a ‘front’ octant are the main exceptions. Methyl substituents at the β-axial positions fall into the two distinct classes, having consignate or dissignate c.d. effects, respectively, according to the number of bonds comprising the primary zig-zag which passes through the particular β-carbon atom. The scope and significance of these conclusions are discussed, and attention is drawn to some apparent correlations between c.d. and 13C n.m.r. data.