Mass effects have been investigated in relation to the chemical physics of detonation in gases. In the normal stable detonation of mixtures of oxygen with hydrogen, effects of dilution with argon or with helium are as expected from conventional theory, Replacement of hydrogen by deuterium leads to a drop of velocity, as expected from the difference in density of the gases and enthalpy of the chemical reactions. But for marginal conditions of detonation, much more marked influences of molecular mass on stoichiometric limits are observed. In the present paper, these are recorded for the stoichiometric mixtures S$_H$ = 2H$_2$ + O$_2$, or S$_D$ = 2D$_2$ + O$_2$, suitably diluted to yield the pairs of systems $S_H + H_2, S_H + O_2, S_H + Ar, S_H + He,$ $S_D + D_2, S_D + O_2, S_D + Ar, S_D + He.$ In all these cases, the SE systems show more extended detonation limits than SD systems. With our present knowledge of gaseous detonation, more than one interpretation can be given to the mass effects observed. These are discussed to throw light on factors controlling the stability of detonation. Possibilities include the influence of mass effects on microturbulence, on the transfer of activation energy, and on Sorer gradients of mass and of electric charge in the front of the reaction zone. A second stable regime has been discovered with lower velocities of propagation; it can be conveniently observed only near the stoichiometric limits for the normal mechanism of propagation in the systems S$_H$ + H$_2$, S$_H$ + O$_2$ and S$_D$ + O$_2$.