High temperature, short residence time pyrolyses of n-decane, cyclodecane, and decahydronaphthalene (decalin) were experimentally investigated. A coiled tube flow reactor was used with outlet temperatures ~1100 K, residence times ~300 milliseconds, and steam/hydrocarbon ratios ~1, to simulate commercial steam cracking at severities from 1 to 3. Decomposition kinetics and product selectivities were interpreted by Rice-Herzfeld types of free-radical pathways and also by graph-theoretic methods based upon the A(djacency) and C(ycle) matrices of the substrates. The profound influence of the size and numbers of rings within each substrate upon their pyrolysis pathways was discerned from the observed yields of specific products, namely, ethylene, propylene, butadiene and benzene, of interest in olefin plant operations.