Over 260 collision cross-section σ o t, expressed in »ngströms squared, have been determined for the studied ions at 20 and 70 eV by extrapolation of σ t to zero target gas pressure, and these yield two types of structural information. The first type concerns occurrence and detection of cyclic ions, the second isomerization of parent molecular ions and different product ion distributions at 20 and 70 eV. In addition, examples of two distinct fragmentation mechanisms operative in the formation of identical daughter ions from a given precursor could be traced. Formation of cyclic daughter ions is, for instance, observed for C 2H 3O + from oxirane, C 3H 5O + from oxetane, C 4H 7O + 2 from 4-methyl-1,3-dioxolane. Cyclic molecular ions are formed in varying proportions from oxirane, tetrahydrofuran, 2- and 4-methyl-1,3-dioxolane but not from porpylene oxide, oxetane and 1,3-dioxolane. Isomerization of the parent molecular ion is proposed for the following fragmentations: CH 2 H + from allyl alcohol, CHO 2 + from formic acid, C 2H 2O ·+ from oxirane, and C 3H 6O ·+ from 3-methyl butanal and 2-methyl pentanal. Different product ion distributions at 20 and 70 eV were found for C 3H 5O + from ethyl propionate and 2-pentanone, C 2H 4O ·+ and C 4H 8O ·+ from butane-1,3-diol, and C 3H 6O ·+ from 2- and 4-methyl-1,3-dioxolane. Two distinct fragmentation mechanisms were traced for the following processes: CH 2 H + OH, C 2H 2O ·+ and C 2H 3O + from methyl vinyl ether, CH 2 H + and C 2H 5O + from butane-1,3-diol and C 2H 2O ·+ from butanone. Self protonation of acetaldehyde also appears to take place by two mechanisms. Energy partitioning is evident in the formation of formyl cations HCO + but wears off for processes in which larger daughter ions are formed. For formyl cations from straight chain aldehydes, the 70 eV collision cross-section is linearly related to the logarithm of the reciprocal of the number of degrees of freedom in the parent molcule, log (1/ DF p). One example of a proton-bound dimer is given, that of acetaldehyde. Its cross-section value is exceptionally high, more than three times than that of its monomer. Such behaviour is probably typical of this type of cation.