Interferometric studies of low pressure laminar detonations have shown the existence of an induction zone with a maximum density in accord with that calculated for the shocked and vibrationally relaxed but unreacted initial mixture. In 2H2+ O2+ 2CO and 7H2+ O2the vibrational relaxation time is short compared to the induction time, and the latter time is in approximate agreement with the data of Schott and Kinsey. In H2+ 3O2the observations suggest that the vibrational relaxation time is comparable to, and may in fact determine, the induction time.