The volumes of reaction determined for the hypothetical cyclization of l-alkenes to cycloalkanes decrease continuously from the formation of cyclopropane (Delta V-R = -5.5 cm(3) mol(-1)) up to the formation of cyclodecane (Delta V-R = -32.3 cm(3) mol(-1)) and seem to be constant for the larger rings. The analysis of the packing coefficients (eta = V-w/V) leads to the conclusion that this ring-size dependent decrease in volume results from the different packing of cyclic and acyclic compounds rather than from the changes in their intrinsic molecular volumes. The investigation of the intramolecular Diels-Alder reactions of (E)-1,3,8-nonatriene (E)-1 and (E)-1,3,9-decatriene (E)-2 leading to the bicyclo[4.3.0]nonenes cis- and trans-4 (Delta V double dagger/Delta V-R [cm(3) mol(-1)] -24.8/-32.0 and -24.8/-28.5, respectively) or bicyclo[4.4.0]decenes cis- and trans-6 (-37.6/-45.4 and -35.0/-37.4, respectively) confirms the ring-size dependence of the activation and reaction volumes. The dependence of the effect of pressure from the number of newly forming rings is illustrated with the thermolysis of (Z)-1,3,8-nonatriene (Z)-1 in which an intramolecular Diels-Alder reaction leading to bicyclo[4.3.0]nonene cis-4 competes with a sigmatropic [1,5] hydrogen shift leading to (E,Z)-1,5,7-nonatriene 7. The use of high pressure causes a reversal of selectivity.