Calculations of detonation velocities in steady-state regimes are insensitive to specific assumptions made about mechanisms of energy liberation. By contrast, researches on conditions for marginal detonation can yield much more information. Recently obtained results are as follows: o (A) Mass Effects of Diluent Molecules. With systems whose reaction kinetics are similar, diluent additives show clear mass effects. Limits for S H =(2 H 2 +O 2 ) diluted with H 2 or O 2 extend further than for S D =(2 D 2 +O 2 ) diluted with D 2 or O 2 . However, dilution with argon extends the limits further for S H or S D , than dilution with helium. One group of explanations of these observations is related to theories for effects of third-body additives in conventional chemical kinetics. Alternative interpretations more specifically related to detonation refer: (i) to the structure of a detonation front in its perimeter of contact with the containing walls; (ii) to charged particle diffusion gradients at the detonation front, leading to “detonation dipoles”; and (iii) to the role of mass flow in promoting activation of the molecules in the unreacted gases. (B) Fluctuating Detonation and Dual Detonation Regimes. Near the limits, reproducible humps are found in the smooth plot of detonation velocity against composition. Possible explanations for such humps include: (i) a spinning detonation head; (ii) the formation of solid reaction products; and (iii) for mixtures of hydrogen with oxygen, the detonation chemistry near the composition limits follows alternative paths, to produce either water or hydrogen peroxide as detonation product. (C) Chemical Effects of Diluent Molecules. Under marginal conditions, chemical reaction kinetics can play a decisive role in extending or curtailing detonation limits. Some compounds have a marked negative effect on the detonation of 2 H 2 +O 2 . (D) Formation of Solid Products in Detonation. Certain ranges of mixtures between oxygen and gaseous fuel molecules such as acetylene, furan, benzene, aliphatic hydrocarbons, or cyanogen, may produce elemental carbon, or carbonaceous polymers. Gaseous molecules such as tetramethylsilane may produce silica or silicon-containing polymers. Certain consequences of the production of such solids are of decisive importance for marginal detonation. A substantial fraction of the total enthalpy change due to the chemical reaction is not available until solid particles can be formed. Compared with conventional combustion reactions, novel features are observed in the detonation chemistry, in mechanisms of energy liberation, and in modes of transfer to the detonation front.